What Is Heat?

Step 1 of 5

Everything is made of jiggling particles.

Even a solid steel bar is full of atoms vibrating in place. They never stop — not in ice, not in a freezer, not anywhere above absolute zero.

Step 2 of 5

Temperature measures how hard they jiggle.

Heat something and its particles move faster. Cool it and they slow. A thermometer is really an atom-speed gauge with numbers on it.

Step 3 of 5

Heat flows from fast to slow.

When fast atoms meet slow ones they knock energy across, like fast balls hitting slow ones. Heat always moves from hot to cold, never the other way by itself.

Step 4 of 5

Cold is not a thing you add.

There is no such substance as cold. A fridge does not add cold — it removes heat and dumps it out the back, which is why the back of a fridge is warm.

Step 5 of 5

There is a bottom, but no top.

At −273.15°C all jiggling stops. You cannot go colder, because there is no slower than stopped. But there is no limit to how fast atoms can go.

Rubbing your hands adds heat by force — you push the atoms into moving faster.