Heat

Heat is a kind of energy that flows from warmer things to cooler things. Heat is caused by the tiny particles inside matter, called atoms and molecules, jiggling around. The faster they move, the hotter something feels. When they slow down, the object cools off. Heat is measured in units called calories or joules, while how hot something is gets measured as temperature.

Every object is made of atoms, and those atoms are always moving. In a cold ice cube, the atoms barely wiggle. In a cup of hot cocoa, they zip around much faster. When you touch the cocoa, some of that energy passes into your hand. That is why the mug feels warm.

Heat travels in three main ways. The first is called conduction. That happens when two things touch, like a metal spoon sitting in soup. The spoon heats up from the end in the liquid. The second way is convection. That happens in liquids and gases when warm parts rise and cooler parts sink, making a slow loop. A pot of boiling water moves this way, and so does the air in your house. The third way is radiation. The Sun warms Earth through radiation, sending heat across 93 million miles of empty space, which is a distance light itself takes about 8 minutes to cross.

Heat can change what matter looks like. Add enough heat to ice, and it melts into water. Add more, and water turns into steam. Take heat away, and steam turns back into water, then into ice. These are called changes of state, and they happen because heat speeds up or slows down the atoms inside.

People once thought heat was a secret fluid called caloric that leaked between objects. In the 1800s, scientists like James Joule showed that heat is really a form of energy, tied to motion. That discovery changed physics and helped engineers build better steam engines, refrigerators, and power plants.

Your body makes heat all the time. Right now, the food you ate is being broken down to keep you near 98.6 degrees Fahrenheit. When you shiver, your muscles twitch fast to make more warmth. When you sweat, water on your skin carries heat away as it dries. Every animal, every engine, and every star in the sky is in the same business: moving heat from one place to another.