Bubbly and Boiling…


What are bubbles?
Bubbles, in general, are globules of one substance in another. Usually, it is a gas in a liquid. It is globular in shape since it has the most stable and lowest energy state. Bubbles collapse when the pressure of the substance inside matches that of outside.

Let us understand how the bubbles are formed in liquids. Let us consider the case of water.

Bubbles in boiling water
We know that boiling water produces bubbles; few at the beginning… then we see those rising… and more and more as the temperature is increased and the water reaches the boiling point… The boiling phenomena is accompanied with a sound.. What causes  all these?

It is a known fact that water has a lot of air mixed in it. When the temperature of the water is raised, this solubility of air in water is decreased and air gets separated out in water. That is what we see as the bubbles at the bottom and sides of a pan of water over fire!

As the temperature is increased, water at the bottom of the pan (close to the source of heat) starts vaporizing into the bubbles. As these water vapours have density less than that of water around them, they rise to the surface of water and try to escape from the water.

It should be noted that the atmospheric pressure (weight of air on the surface of liquid) acts against the vapour escaping water when the vapour pressure is low. At temperatures less than the boiling point, the vapour pressure is less than the atmospheric pressure. So, at a temperature lower than the boiling point of water, the bubbles may rise, but they won’t reach the surface of water. It cools down on its way up and collapses. The sound we hear prior to the boiling water is nothing but the sound of collapsing of bubbles.

At the boiling point, when all the liquid molecules have the willingness to vaporize, they form bubbles rapidly. The vapour pressure at boiling point will be higher and equals the atmospheric pressure. So, the bubbles rise to the surface of water and burst releasing the vapour to the surrounding.

Now, a question: What causes water to boil faster at higher altitudes?
Answer: At higher altitudes, the atmospheric pressure is less than normal. So, the vapour pressure of the boiling water will be equalling the atmospheric pressure at a lower temperature and vapour escapes from water. So, boiling point will be lower at higher altitudes.


Vaporization vs Evaporation vs Boiling


While working at my previous organization where I was a development editor for school books, I had quite a lot of interaction with school teachers. The topics of these interactions vary from new teaching methods to the content which goes into the text books to various other subject related matter. Mostly, the discussions were on their views regarding the explanations of certain “confusing” topics.

Teachers can be intransigent sometimes. Teachers, especially the experienced ones, usually follow only those books they have started their teaching career with. So, they get upset if the new content in our books deviate slightly from the wordings they are familiar with, or the artwork or diagrams they are used to. There were certain topics where the rewording confused the teachers and they ended up confusing us with their corrections. A set of topic I came across while working on the Physics books was vaporization/evaporation/boiling. These terms drove my series editor to a confused state. She sent different sets of corrections. I managed to “negotiate” with this middle school teacher and a general explanation was given for the book. But I realized, looking up in internet, many people are confused as well: the difference between vaporization, evaporation and boiling. So, in this post, I briefly discuss what these are and how they differ from each other.

Vaporization: It is nothing but the change of state of matter from a liquid state to a gaseous state. For example, change of water to steam. This change of state from liquid to gas can happen by two methods: evaporation and boiling.

Evaporation: It is the change of state of liquid to gaseous at a temperature below the boiling point of the liquid. There is no boiling of liquid involved in evaporation. Evaporation takes place at the surface of liquid which is exposed to the surroundings.

Boiling: When the liquid is heated and attains the boiling point, the vaporization process is called boiling. Boiling happens throughout the liquid. This is because, the heating process (convection) heats up the entire liquid.

Let us understand these better:

What causes a liquid to change its state to gas?
We know that the various states of matter are characterized by the arrangement of molecules in them and also the intermolecular force of attraction.

In solids, the intermolecular force of attraction is strong. That is why the molecules are held together tightly and the matter is rigid.
In liquids, the intermolecular force of attraction is less, which gives the matter the ‘fluid’ characteristics.
In gases, the intermolecular force of attraction is least. That is the reason, they don’t usually hold together as a substance and easily spread everywhere.

For a solid molecule to change its state to liquid, it should have enough kinetic energy to overcome the intermolecular force of attraction which binds it to other molecules. You know that heating can provide that kinetic energy. Similarly, heating provides enough kinetic energy to a liquid molecule to overcome the intermolecular force of attraction and change to gas.

During evaporation, the liquid absorbs heat from the surroundings. The molecules at the surface of the liquid acquire enough kinetic energy and get converted to vapor and escape. The more heat the liquid can absorb, the more will be the rate of evaporation: On hot summer days, we see the water in the pond getting evaporated faster than other seasons.


Evaporation of lake water

When we apply heat externally, for example, heating up a pan of water, the kinetic energy of the molecules increases rapidly and they start to vaporize. There will come a point at which all the molecules in the liquid have enough kinetic energy to change their state to gas. This is called the boiling point and the process is called boiling. Vaporization during boiling will be quite rapid.


Boiling of water

Vapor Pressure
Another factor which differentiate these processes is vapor pressure. It is the pressure exerted by the liquid on the walls of a container. Liquids which can vaporize easily will have a higher vapor pressure. During evaporation, the vaporization is a slow process and the vapor pressure will be less as compared to the atmospheric pressure.

During boiling, at the boiling point, the vaporization is fast and the vapor pressure is equal to the atmospheric pressure. That is the reason, the bubbles formed (with vapor inside) during boiling rise up and burst at the surface of the liquid.

Summing up, both evaporation and boiling are same kind of phenomenon where the liquid change its state to gas. The difference depends on the supply of heat: Evaporation can take place at any temperature while boiling happens only at the boiling point. Another difference is, evaporation takes place only at the surface of the liquid while the boiling is a bulk process where vaporization takes place throughout the liquid. Also, during evaporation no bubbles are formed, but, bubbles are formed during boiling.


Evaporation and boiling

So, the concluding question: What causes the formation of bubbles during boiling? That calls for another blog post.

The levers in our body


Archimedes once said, “Give me a place to stand and I will move the Earth.” It was Archimedes who came up with the idea of levers which will reduce our effort in moving heavy load. Levers are simple machines which makes our lifting and moving of things easier. We use a variety of levers in our daily life to make many tasks done with less effort.

A lever works around a fixed point called pivot or fulcrum (F). It is a point about which the lever moves. Load is the weight to be lifted or moved using the lever and is represented by L. Effort is the input force applied by us and is represented by E. Levers are classified into three based on the positions of the Fulcrum, Load and Effort.

Class 1: Fulcrum between Load and Effort. Examples include scissors and wire cutters.

Class 1 lever

Class 1 lever

Class 2: The load is between fulcrum and effort. Examples include bottle opener and nut cracker.

Class 2 lever

Class 2 lever

Class 3: The effort is between the fulcrum and the load. Everyday example include thongs, tweezers and staplers.

Class 3 lever

Class 3 lever

Working of levers is based on the components fulcrum and effort to balance the load. We consider our body as a complex machine with a number of simple machines working together. The bones, muscles and the many joints in our body form different types of levers for us to lift and balance weight or load. Imagine lifting a weight without the use of your joints and standing on your toes without an effort by your muscles. Human body has all the classes of levers.

Class 1 lever: Move your head up and down feeling the muscles at the back of your neck. The movement of the head is supported by these muscles and the head moves around a point where the top of the spine meets the skull. This forms the fulcrum of a class 1 lever. Here head is the load and the muscle at the back of the neck which supports the skull up is the effort.

The movement of head is based on Class 1 lever

The movement of head as a Class 1 lever

Class 2 lever: Stand on your toes, and we can feel the calf muscles stretching up to support your weight. When you stand on your toes, your toes act as a fulcrum. Your calf muscles provides the effort to balance the weight (load).

Standing on toes is based on Class 2 lever

Standing on toes as a Class 2 lever

Class 3 lever: Lifting things requires bending your arm. Your arm works as a class 3 lever. The fulcrum is at the elbow. The biceps muscles provide the effort to bend the forearm against the weight of the forearm (load). If you are bending your arm with some weight in hand, that weight adds to the load.

Lifting weights or bending our arm is based on Class 3 lever

Lifting weights or bending our arm as a Class 3 lever

We maintain the tools and machines we use every day with great care: oiling them and use them regularly so that they work smooth. In the same way, it is necessary to maintain the simple machines in our body. Give the joints and muscles the exercise they need every day. Work out, walk, run,… stay active… never be idle because the body machinery will become less efficient without being charged up!!!

Static Electricity

static el 10

Have you ever wondered what causes a bad hair day or the sparks from your woollen blanket during winter or the random shocks you get from random objects now and then or the standing fur of your pet cat or the most obvious, disastrous lightning dashing across the evening sky!? There can be one answer to all these – Static electricity!!!

The general definition of static electricity is that it is an imbalance of electric charges in a material. The material remains charged until it can discharge these charge (electric current) trough any medium.

Static electricity effects are evidently annoying. However, it should be said that life is impossible without the forces associated with static electricity. Static forces, both positive and negative, hold the world of atoms and molecules together in the right balance. But, what is static electricity? Is it really harmful? Why it is more during winter? To understand the nature of static electricity, one should know what things around are made of.

Atomic Structure
We know that all material objects are composed of atoms (elements). These elements combine to form compounds (molecules). Protons, neutrons and electrons constitute the sub-atomic particles. The massive (comparatively) protons (positively charged) and neutrons (neutral in charge) are located at the centre (nucleus) of the atoms which forms a dense positive core, while ‘electron shells’ which are concentric spherical regions with distinct energy levels are the home of the negatively charged, mass-less electrons.

Electrons are loosely bound in the atoms while protons and neutrons are tightly bound inside the nucleus. A charged atom: A neutral atom has equal number of protons and electrons: i.e., equal number of positive and negative charges. Provided with sufficient energy, electrons can be freed from its attraction to the nucleus, thus removing them from the atom. Electrons from different atoms can enter into the electron shells of an atom. This entering or leaving of electrons makes an atom charged due to the imbalance of positive and negative charges in it. This charged atom is referred to as ion. Ions can be positive (cation) or negative (anion) depending on electrons leaving or entering an atom.

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Formation of cation and anion from a neutral atom by loosing and gaining electron(s) respectively

Transfer of charge
What happens when we have a charged object? Every charged object tends to redistribute the excess charge across its surface by electron movement. This movement of electrons can take place freely only in the case of a conducting body. When a conducting body is charged from a point, the body will distribute the charges across its entire surface.

Additional charged transferred to a conductor getting evenly distributed across its surface of the conductor

Additional charges transferred to a conductor getting evenly distributed across the surface of the conductor

This redistribution of charges can be explained by the basics of charge interaction: unlike charges attract and like charges repel, thus making the excess charges spread across the entire surface away from each other. It should be stressed that additional charges, positive or negative, is redistributed only through the movement of electrons. While additional negative charges (excess of electrons) get distributed by the movement of electrons itself, additional positive charges (deficiency of electrons) attract electrons from elsewhere creating positive charges wherever the electrons have come from.

Like charges attract and unlike charges repel

Like charges repel and unlike charges attract

Conducting human body
We know that human body is a good conductor of electricity. When charges are transferred human body, they spread across the entire body even through the strands of hair. When the hair gets charged (like charges), they repel each other making them rise upward and outward separated from each other as possible. This is the common ‘hair-rising’ phenomenon of static electricity.

Why static electricity effects more during winter?
Shocks from door knobs, bad hair days, etc. are frequent during winter compared to summer. Why? This has to do with the humidity which affects static charges. Water molecules can remove the extra charges in a material. We often fix our hair with water! Winter is the driest of all seasons with little moisture in air and the static charges remain more on the surfaces of the body often getting discharged via static electricity.

Polarization of charges
We know that like charges attract each over and unlike charges repel each other. But what about the interaction between a charged and neutral particle? There is always an attractive force between a charged (positive or negative) and neutral particles. This is due to the polarization of charges in a material. Polarization is the process of separation of charges within a material; i.e., the positive and negative charges get separated forming two poles at either ends as shown in the following image:

Polarisation of charges

Polarisation of charges

Consider the classic example of a charged balloon held against the flowing water from a tap;  the stream of water gets attracted towards the charged balloon. In the vicinity of the charged object (negatively charged balloon), the neutral water molecules will get polarized: Hydrogen as the positive pole and oxygen as the negative pole. Molecules in a liquid are free to move. Hence, the water molecules get realigned and the positively charged oxygen molecules move towards the negatively charged balloon, pulling the entire steam of water with it thereby deflecting its flow.

Polarisation of charges in water causing a deflection of water stream

Polarisation of charges in water causing a deflection of water stream

Charging of objects
So far we have discussed the effects of static charges and charge interactions. Now, let us look into how these charges are created in objects. Charging can be done via various methods such as conduction, induction, friction, etc.

The word conduction  means two objects in contact. Charging by conduction takes place when a charged object is brought in contact with a neutral object, transferring the charges from the charged to the neutral object.

Charging by conduction

Charging by conduction

Charging by induction takes place when the charged object is brought in the vicinity of the neutral object (no contact). This causes a polarization of charges in the neutral object and with proper earthing, one can remove one type of charge from it making it charged.

Charging by induction

Charging by induction

Charging by friction  is the simplest way of charging objects. This is done by rubbing the objects together. As objects of different materials are rubbed together, electrons with weaker bonds (in least electron loving material) are removed from one material and migrate to the other material making both materials charged. Friction is usually the reason behind the static charges getting accumulated on our woollen clothes (when rubbed against us), cat’s fur (when they move about the carpets and us), the balloons (when rubbed against other substance), and so on.

When a glass rod is rubbed on a silk cloth, the glass rod loses electrons to sink, making the silk cloth negatively charges and the glass rod positively charged

When a glass rod is rubbed on a silk cloth, the glass rod loses electrons to sink, making the silk cloth negatively charges and the glass rod positively charged

Lightning is one of the dangerous effects of static electricity. It is the discharge of the static charge built up in the clouds. There is a polarization of charges in the clouds; the top parts acquire positive charges while the bottom parts acquire negative charges. This static charging of the clouds is believed to be due to frictional charging.

Frictional charging of clouds
Clouds contain millions of ice particles and water particles suspended in them. When additional water from the ground gets evaporated and rises up forming droplets on its way up towards the cloud, the upward motion of moisture collides (frictional) with the water droplets already suspended in the cloud. During these collisions, electrons are ripped off from the rising droplets causing the bottom of clouds to be negatively charged  and the positively charged droplets rises up to the top of the clouds making the region positively charged..

Lightning strike
The polarization of charges in the cloud, with the electrons gravitating towards the Earth’s surface causes the Earth’s surface to be charged positive. As the build up static charge in clouds increases, there is an increased electric field in the atmosphere surrounding the cloud making it highly conductive. This makes way for an easy path for the transfer of charges from the clouds to the surface of Earth. Since Earth’s atmosphere has numerous dust particles and other impurities, there wont be a straight line transfer of charges from the cloud to the Earth: The reason behind the zig-zag motion and branching in a lightning. Also, the purplish glow of the lightning is a characteristic of the ionized air molecules.



Lightning is not a discharge from the cloud to the Earth. It can be between cloud to cloud or even within a cloud because of the polarization of charges in the clouds.

Various kinds of discharge from clouds

Various kinds of discharge from clouds

The sudden discharge from the clouds heats up the surrounding air which expands violently. This creates a shock wave which we hear as thunder.

A note on Electric Heating…

Heating coil in an oven.

Heating coil in an oven.

Somebody asked me last day about the relations between electrical resistance and heating and light emission. So I thought i would rather give a short note on resistive heating as a reply. Here is a ‘not so short’ version of that reply:

What is Resistance?
We know that (electric) current is the flow of electric charges in a circuit. In electric circuits, this charge is carried by the moving electrons through a wire driven by the potential difference between the ends of the wire. Often, an electron moving through an electric circuit encounters hindrance to its flow due to its many collisions with the atoms in the conducting wire. This is called resistance. There are numerous factors which causes this resistance.
  • First is the length of the wire. The longer the wire, more are the chances of collisions and hence, more resistance.
  • Second, the cross-sectional area of the wire; thinner wires offer more resistance to the flow (just like a thinner tube providing more resistance to the flow of water through it compared to a wider tube).
  • Third, the material used. Some materials offer less resistance to the flow of charge. Meaning, they are better conductors. Examples include copper and aluminium used in household circuits
Ohm’s Law
Ohm’s law states that the current through a conductor is directly proportional to the potential difference between the two ends. The law can be written in terms of resistance as,
I = \frac{V}{R},
where I is the current through the conductor and V is the potential difference measured across the conductor, R being the resistance of the conductor. The equation says that greater the voltage supplied to the circuit, greater will be the current flowing through and larger the resistance, lesser will be the current flowing through.
Joule Heating
Now, let us come to the heating part. We just saw that materials or elements with resistance (or simply resistors) oppose the flow of electric current. So, the electrical energy needs to push the current through the material which results in the current converting the electrical energy to heat. This is called Joule heating or ohmic heating or resistive heating.
We know that heat is the kinetic energy of the particles as they vibrate and collide with other particles. With an increase in voltage, for a material with high resistance, there will be an increased collision of the flowing electrons with the atoms in the material. This results in the heating of the resistor. The amount of heat released is given by
Q \propto I^2 \cdot R
This dissipation of electrical energy is often undesired, like in the case of transmission losses in power lines. On the other hand, Joule heating is sometimes useful: examples include electric stoves and other electric heaters. The electric heaters are also called resistive heaters.
Another example of resistive heating is incandescence. Incandescence is the emission of light from a hot body (in general, emission of electromagnetic radiation from a hot body). Incandescent lamps and the electric heaters rely on Joule heating: the filament is heated to such a high temperature that it glows ‘white hot’. Higher the temperature the material attains, the brighter it will be due to the increased emission of the EM light which in turn is due to the increased vibration and collision of charges.
Incandescent light bulb.

Incandescent light bulb.

Convective Heat Transfer

Recently I have been reading about ‘Energy’, especially the heat and light energies and the transfer of both these forms of energies which basically sustains life on this planet. This post discusses a mode of heat transfer, i.e., convection which explains many of the everyday phenomena around us.

Heat is transferred by three basic modes:

  • Conduction
  • Convection
  • Radiation

Conduction of heat occurs through a thermally conductive material such as metal, or between two thermally conductive materials in contact. Radiation is the transfer of heat from one body to another when they are not in contact. Here heat is transmitted through space in the form of electromagnetic radiation in the infra-red region.

Convection is the method of heat transfer via the mass motion of fluid; liquid or gas. When fluid is heated, it moves up from the source of heat to cooler areas carrying the heat energy and its space is filled by cooler fluid from the surroundings which in turn gets heated up and starts moving up, continuing the process and transferring heat energy from the source of heat to the surroundings.

Convection explains many of the day-to-day phenomena such as:

  • Boiling of water
  • Fire heating up the surroundings
  • Wind
  • Cloud formation, thunderstorms, etc.

Let us looking into few of them to understand heat transfer process.

Convection in Liquid
We know that heat is the result of vibration of atoms and molecules in an object, often bumping into each other when triggered by an energy source.
Consider boiling of water. Water is heated from the bottom of a pan, close to the heat source. It should be noted that the pan gets heated up via conduction; first the bottom of the pan is heated up by the source (which subsequently heats up the water above it) and the heat is transferred to the rest of the pan via conduction. When water at the bottom gets heated, the particles move faster and move farther apart making it less dense.

 When particles move farther apart, their volume increase. Since mass does not change for a given volume of water, the density decreases.

The less dense water rises up to float over the cooler, denser water. As the warm water rises up, its place is taken by the cooler water from the surroundings which then gets heated up and rises up. By then, the warm water which has already risen up to the top, gets cooled by the surroundings and moves down with the new warmer water displacing it. This eventually leads to a continuous circular motion of water in the pan known as convection current.

Convection current in water...

Convection current in water.

Note that the cooling of water when it reaches the top of the pan is also via convection. The warm water at the top transfers heat to the air above it which rises up, and that space is filled by cooler air which then gets heated up and rises as well, providing a continuous supply of heat transfer upwards. This continues till the heating stops and water in the pan cools down completely.

As you place your hand well over a boiling pan of water, you can feel the heat rising.

Newtion’s law of cooling
The rate of heat loss of a body is proportional to the difference in temperature between the body and the surroundings.
Example: Cooling of a hot object when placed in a cooler surrounding.

Cooling of a hot solid via convection...

Cooling of a hot solid via convection.

Atmospheric Convection
Atmospheric convection results in wind, cloud formation, thunderstorms (moist convection), etc.

  1.  Wind
    Wind is the movement of air from a region of high pressure to a region of lower pressure. Low pressure systems are the result of the uneven heating of the Earth’s surface.
    For example, land and water bodies are heated unevenly during day and night. That explains the directions of land breeze and sea breeze often observed in the coastal areas.
    Day time:
    During day time, land gets heated up faster. This causes the air above the land to heat up and rise up creating a low pressure system there. The air from the cooler areas (sea/ocean) fills that space which again gets heated up and rises up and repeating the process we have a circulation of air from sea to land. That is, during day time, the wind blows from sea to land.
    Night time:
    During night time, the land gets cooled down faster than the water body. This means that the water body is warmer than the land which causes the air above the water to heat up and rise up. Then a low pressure system is created above the water, causing air from land to move to that space which in turn gets heated up and rises up causing an air circulation from land to sea. That is, during night time, the direction of wind changes.

    The directions of winds during day and night are different due to the uneven heating of land and water.

    The directions of winds during day and night are different due to the uneven heating of land and water.

  1. Cloud formation and Thunderstorms
    Cloud formation and thunderstorms are the results of moist convection. This happens in areas where there is moisture in air.
    When the Sun’s rays heat up the land, it causes the air above it to get heated up as well. This warm air rises up accompanied by the moisture in air. As it moves up, it starts getting cooled down as the atmospheric temperature drops. When it reaches a certain altitude, it gets saturated and the moisture starts to condense to tiny droplets and expand.
    As it moves further up, temperature further drops and the condensation and expansion continues. These condensed tiny droplets are suspended in air and when they come together, clouds are formed.

    Cloud formation

    Cloud formation

    In tropical areas, where there is a high content of moisture in air, the cloud formation is accompanied by rain when the accumulated water droplets become heavy and fall under gravity.
    We also know that the Sun’s rays heat up water bodies, causing vapour to rise. This vapour in air is what we call humidity (moisture).

    Thunderstorms occur in tropical areas with high humidity. During the cloud formation, the air cools and condenses to tiny droplets. During the condensation process, the air releases heat inside the cloud causing it to get warmer as compared to its surroundings. This keeps the air unstable, increases the buoyancy of air and convection continues. This results in the building up of clouds vertically upwards. When it reaches a certain altitude where the temperature is below freezing, large raindrops, ice crystals and hail are formed.
    When these ice crystals and hail are heavy enough, they fall under gravity as precipitation. This causes a cool drift of air downwards. When this collides with the warm moist updrift, lighting and thunder will occur.
    And what causes lightning? We will discuss that in another post!!!

Basics: Where does one start?

Few months back someone tested my knowledge in basics… I was keep on talking about my research field in particular and solid state physics in general… and then came questions on some basic stuff…

I was a difficult moment as i struggled to answer few of those really basic questions… That day i decided, it’s time to go back to basics…
But where should i start?
I started on some of the university Physics books… These days you can get all those bulky ones downloaded… Started with Crowell’s Conceptual Physics, Young and Freedman’s University Physics with Modern Physics… and Halliday’s Fundamentals of Physics… and few others in the queue… and ofcourse Feyman’s lectures to read when bored of the other books…
It felt so good to get the concepts refreshed in your brain… i admit i had forgotten many of the laws and equations… So, i had advised many of my colleagues and collaborators about the mentioned list of books if they lose track of basics… 
But recently, I had experienced a problem of different kind… I was preparing myself for entering into the K12 education industry… I was always fascinated by them as they come up with a variety of learning aids every year for science… It needs a lot of creativity and i was eager to jump into that and learn few techniques…
I was going through some high school level science books… The books i read had few mistakes, but it surprised me with the amount of information it provided… I don’t remember how i passed my high school science… but the books i read blew me away; it made me dig deeper for more basics… I could refresh my knowledge in other areas of science and how they are all related… I surprised myself by taking short notes… I feel so excited right now… I have listed down a few more basic books to read for the next few weeks; few high school books included with numerous other college level books… 
Usually after post doc experience, many researchers and academicians go and settled down in teaching… But unlike them i chose to stay away from it… that caused a decline in my understanding of basics because i never get to teach anyone… Getting a PhD doesn’t mean you have learned everything… If you don’t use (share/teach) what you have learned over years, there is a possibility you tend to forget things…. So, i have to occasionally go back to books, and from now on, i have decided never stop learning; probably something new every single day… 
And my advice to everyone: Find yourself weak in basics? Go back to school… It might be an embarrassing story for some to tell… but its worth it… 
Now, I can wait for my new role… Educationist!!! Sounds so exciting….