The Universe is made up 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 very 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 example, when we are cold, we can jump up and down to get warmer.

Here are just a few examples of various types of energy being converted into thermal energy (heat).

(1) Mechanical energy is converted into thermal energy whenever you bounce a ball. Each time the ball hits the ground, some of the energy of the ball's motion is converted into heating up the ball, causing it to slow down at each bounce. 

A thermal infrared image of a ball before (left) and after (right) being bounced.

(2) Thermal energy can be transfered to other objects causing them to heat up. When you heat up a pan of water, the heat from the stove causes the molecules in the pan to vibrate faster causing the pan to heat up. The heat from the pan causes water molecules to move faster and heat up. So, when you heat something up, you are just making its molecules move faster.

(3) Electrical energy is converted into thermal energy when you use objects such as heating pads, electrical stove elements, toasters, hair dryers, or light bulbs.

A thermal infrared image of a hair dryer and a flourescent light bulb.

(4) Chemical energy from the foods we eat is converted into heating our bodies.

(5) Light from the sun is converted to heat as the sun's rays warm the earth's surface.

(6) Energy from friction creates heat. For example when you rub your hands, sharpen a pencil, make a skid mark with your bike, or use the brakes on your car, friction generates heat.

A thermal infrared image of a pencil after being sharpened (left) and of hot brakes in a car (right). Notice the hot tip of the pencil.

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.

Summary: Heat is the energy an object has because of the movement of its atoms and molecules which are continuously jiggling and moving around, hitting each other and other objects. When we add energy to an object, its atoms and molecules move faster increasing its energy of motion or heat. Even objects which are very cold have some heat energy because their atoms are still moving.