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We know now that heat is not a fluid, nor a substance at all, but is really one of the several forms of energy. Other common forms are electrical energy, radiant energy, chemical energy, and the mechanical or kinetic energy possessed by moving materials and objects, such as falling water or a rotating wheel. Atomic and nuclear energy are still other forms. Heat is often labeled thermal energy.

According to molecular theory, all substances are made up of molecules, and as long as the temperature of the substance is above absolute zero (-469°F), these molecules are in some degree of motion. When heat is applied to a substance, the molecular activity increases and the temperature rises. Conversely, if a substance loses heat, the molecular activity decreases and so does the temperature. We can think of heat, then, as the energy of molecules in motion.

Energy cannot be created or destroyed in any way that we know. But it can be changed from one form to another, and this transformation is constantly going on, both in nature and through the activities of man. For example, radiant energy from the sun is transformed by the process of photosynthesis to stored chemical energy in vegetation. When the vegetation is burned, the chemical energy is transformed to thermal energy, radiant energy, and the kinetic energy in the rising air in the convection column over the fire. The kinetic energy in falling water can be used to generate electrical energy, which in turn can be changed back to thermal and radiant energy by an electric heater, or to kinetic energy again through the use of an electric motor.