How does friction turn motion into heat?

Friction is a force that resists relative motion between surfaces, and it is one of the main ways energy is transformed in everyday life. At the microscopic level, rough surfaces interlock and deform, and atoms exchange energy as objects slide. Some of the organized motion becomes random molecular motion, which we experience as heat. This is why brakes get hot, why hands warm up when rubbed together, and why machines need lubrication. Friction is not always bad; it is what lets us walk, grip objects, and write with a pencil.