The universe is made of matter and energy. Matter is made up of atoms and molecules (groupings of atoms) and energy causes the atoms and molecules to always be in motion either bumping into each other or vibrating back and forth. The motion of atoms and molecules creates a form of energy called heat or thermal energy which is present in all matter. Even in the coldest voids of space, matter still has a small but still measurable amount of heat energy. Energy can take on many forms and can change from one form to another. Many different types of energy can be converted into heat energy. Light, electrical, mechanical, chemical, nuclear, sound and thermal energy itself can each cause a substance to heat up by increasing the speed of its molecules. So, put energy into a system and it heats up, take energy away and it cools. For ex ample, when we are cold, we can jump up and down to get warmer.
The more energy that goes into a system, the more active its molecules are. The faster molecules move, the more heat or thermal energy they create. So, the amount of heat a substance has is determined by how fast its molecules are moving, which in turn depends on how much energy is put into it.