This is a neat, and very quick little toy that I first came across on the "Happy Scientist" website.
A couple of minute's work, and you have a toy to keep the kids happy, a fun craft activity, a hook for a fascinating science lesson, and probably the cheapest and quickest at that.
Step 1: Materials and tools
All you need is a flat, thin piece of wood and something to carve it with.
I have successfully used lollipop sticks, tongue depressors, coffee stirrers and chip forks.
I guess it doesn't have to be wood, but it does need to be fairly rigid.