Heat is the form of energy in moving molecules. We feel heat as a change of temperature on our skin and in the air around us. You learned previously about combustion - burning - which produces heat. You also learned that any time energy changes form, heat is produced (remember the Second Law of Thermodynamics?).

Another way heat is produced is through friction. When two or more things rub or move against each other, the molecules move faster, and heat is created. The more the molecules of a substance are moving, the more heat energy it has, and the warmer it is.

It all comes down to the movement of molecules.