Now consider an eraser on a sheet of paper. You rub the eraser back and forth, fast.

You touch the eraser, and it is warmer.

You touch the paper, and it is warmer too.

When two things rub together, both get warmer.

Where did the warmth come from? The rubbing. The eraser and the paper are both warmer.

Now consider a stick and a log. A boy rubs the stick on the log, fast.

The stick warms up. The log warms up where the stick rubbed it.

People once made fire this way. Rubbing for a long time made the wood very hot.

Where did the warmth come from? The rubbing. The stick and the log are both warmer.