|What is heat? — PM, Princeton, NJ
Heat is thermal energy that's flowing from one object to another because of a temperature difference between those two objects. Whenever an object contains thermal energy—which it always does—the atoms and molecules in that object are jittering about microscopically. Each atom or molecule isn't completely stationary; instead it is vibrating back and forth, and pushing or pulling on its neighbors. The object's thermal energy is the sum of the tiny kinetic and potential energies of those atoms and molecules as they move back and forth (kinetic energy), and push or pull on one another (potential energy). The hotter an object is, the more thermal energy each of its atoms has, on average, so this thermal energy tends to flow to a colder object when you touch the two objects together. When that thermal energy is flowing from the hotter object to the colder object, we call it "heat."