Heat — in our everyday language, in physics, specifically — really means internal energy, or the random motion of particles in matter. To put it more simply, the hotter the object, the more its particles move randomly. In physics, heat technically only refers to energy moving from a hotter object to a colder object. We know that heat always flows from an object that’s hotter to one that’s colder, but why can’t it happen in reverse? A hot cup of coffee gets cold because it transfers heat to its surroundings.