Intuitively, when we look at something, we know if that a “thing” is a solid, liquid or gas. But what’s happenning at the molecular level in these three different states (of matter, as we say)?
Understand kinetic energy of molecules in a solid, liquid and gas.
Place the marbles in the tray and tilt the tray to one side. The marbles will arrange themselves in a orderly fashion, similar to molecules in a solid. There is very little movement of the marbles, and therefore the molecules of a solid are said to have very low kinetic (“movement”) energy.
Next, start shaking the marbles slowly. You have now imparted a little of energy to the molecules and they now start moving more easily. They are still attracted to each other so they “coalesce” together, but the extra kinetic energy, and reduced force of attraction causes them to “flow”. You now have the model of a liquid in your hands.
Finally, start shaking them vigorously. Now you have imparted a lot of kinetic energy to the molecules which now start moving around a lot more, and in a completely random manner. This is now a model of a gas.
In all states, the molecules remain unchanged. But their kinetic increases from solid -> liquid -> gas.
Thoughts to ponder
How can we increase, or decrease kinetic energy of matter?
By heat. As you heat something, you make the molecules vibrate faster. Thermal energy is being converted into kinetic energy. The reverse is true too - you can take away the kinetic energy from the matter by placing it in a cool place, i.e, by freezing it.
Five states of matter
Although, the above states are the most common, there are two more - plasma, and Bose-Einstein condensate. Understanding both these states is a bit beyond the current experiment, but hey, read about it anyway.