In the natural world, we are used to seeing substances like water exist in three different forms: solid (ice); liquid (water); gas (steam). The gaseous state generally exists at high temperature,above the boiling point of the liquid, but there are circumstances under which a gas exists at a temperature lower than the boiling point, such as in this experiment. So the atmosphere often contains water vapour, even though the temperature is less than 100 degrees Celsius. In this condition, water molecules may spontaneously collect together and form droplets, e.g. fog, rain, or even, under extreme conditions, ice crystals or snowflakes.
The existence of the different phases is the result of a competition between the attractive forces between molecules and their kinetic energy. The first of these pulls the molecules together and the second encourages them to fly apart. A change of state from a liquid to a gas is called evaporation, and from a gas to a liquid it is called condensation.

Changes of state, such as between liquids and gases, are accurately described by the science of thermodynamics. The attractive forces between molecules contribute to a property known as enthalpy, and the tendency of molecules to fly apart is described by a property known as entropy. The difference between enthalpy and entropy (multiplied by temperature) is called free energy. If we calculate the free energy difference between a liquid and a gas as a function of temperature, we find that when the free energy difference is exactly zero, then a change of state will occur. Just above this critical temperature the system is a gas, just below it it is a liquid.