Heat Thermodynamics Enthalpy Entropy Thermo. Laws First Law of Thermo. Second Law of Thermo. You could burn things, there could be chemical reactions, you can even rub things together and they heat up because of friction.
Thermal energy is measured in calories.
For example, when you burn wood, you release 3000 calories for each gram of wood. When you burn an apple it's only 600 calories. The amount of energy released is directly related to the amount of energy stored up in the chemical bonds. That idea means that there is more energy stored in the bonds of a piece of wood than there is energy in the bonds of an apple. We just talked about friction, heat is also created because of inefficiency.