The Speed of Tiny Things

Temperature is not what most people think. It is not "how much heat something has." It is the average speed of the molecules bouncing around inside a substance. Heat up your coffee and its water molecules jostle and move faster. Chill it down and they slow.

Temperature measures the average kinetic energy of all those vibrating, colliding, ricocheting molecules. Higher temperature simply means a faster average molecular speed. This single idea governs everything from weather patterns to how engines are designed.