Yogurt-lid spinners

Find three or four clean plastic yogurt or sour-cream lids of different sizes. Put them on a smooth surface — kitchen floor, table, tray.

Pinch a lid between thumb and middle finger, flick it. It spins. Watch it.

First attempts fail; the lid scoots, doesn't spin. Within a couple of minutes most 4-year-olds figure out the wrist motion. The discovery is the whole activity.

Add a stopwatch ("how long can you spin it?"). Mark one lid with a sticker so the rotation is visible. Spin two at once and see which stops first.