Why does a paper windmill spin when you blow on it?

A paper windmill can sit perfectly still on its stick. Then you give it a puff, and its bright paper parts whirl around. Why does a paper windmill spin when you blow on it?

The answer starts with air, the invisible stuff all around you. Air fills the space around your windmill, too. You cannot see it, but you can feel it moving when a breeze brushes your cheek.

When you blow, you send air out of your mouth toward the paper windmill. Moving air can give things a push. A puff can move a feather across a table because the air pushes against it.

Your windmill has blades, the folded paper parts that catch the moving air. The blades reach out from the middle like petals around a flower. Their paper is light, so a small puff can move them.

The maker folds the blades so they tilt the same way. That tilt lets your breath push the blades around the middle.

Before you blow, the maker puts a pin through the paper and into the stick. The pin holds the paper with room to turn. The pin is sharp, so the maker handles it.

Now a puff of air reaches the tilted blades. Can you guess what happens next? Your breath pushes the blades sideways, and they spin around the pin.

Each blade catches some moving air. Your breath pushes all the blades around in the same direction. Their little pushes work together.

As you keep blowing, more moving air reaches the blades and keeps pushing them around. When you stop, you stop giving them that push. The windmill may coast for a little while, then slow down and rest.

Air, folded blades, a pin, a sideways push, and a turn around the middle. Your paper windmill spins when you blow because your breath pushes its folded blades around the pin.

Your little puff can make a bright paper whirl, and now you know its gentle secret.