The kinetic theory of matter defines heat as the transfer of thermal from one substance to another substance

Suppose you have a large sealed plastic bag filled with a gas so that it is all puffed up at room temperature. If you cool the bag you will see the bag shrink. This is called . Likewise, heating the bag will cause it to expand and get bigger.

The kinetic theory of matter says the particles within the bag slow down when it is cooled. This means less particles on average will collide with the walls of the bag causing the bag to shrink, and the opposite happens if the bag is heated.

Cooling is the process of removing kinetic energy from the gas particles and heating is the process of adding kinetic energy to particles. All this energy is called thermal energy, but the thermal energy transferred either into or out of the particles is called heat.