Why does heat make paper curl up?

There is a certain amount of water in all ordinary things. The amount of water in a sheet of paper makes it heavy. We know how heavy and limp a piece of quite damp paper is. If we heat the paper, we drive the water away, because we change it into gas that passes off into the air. Spaces in the paper that were filled with water before are now filled with air, which is very much lighter. If there are any forces in the paper tending to make it curl up, they are now free to act, for they have not to lift the weight of the water that was previously in the paper.

Also, we must remember that paper, like almost anything else, shrinks, or contracts when it is cooled, and expands, or takes up more room, when it is heated. If the change in temperature occurs at exactly the same rate in every part of a thing, then its size alters, its shape will not; but if one side of a thing gets hot or cold at slightly different rate from the other side of it, then its shape will change. And if it is a thin, flat thing we shall notice the change by finding it curling up.

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