Rattleback

Take an ancient Celtic axe, place it on a flat surface, spin it, and you'll find two surprises. Firstly, it doesn't behave as you'd expect. Secondly, you have an ancient Celtic axe? Really

No, I thought not. This week, we'll use household junk to make a rattleback, something with the same weird dynamic properties as a Celtic axe. It's a bit less good at chopping your leg off, but in most circumstances that's a plus.

What you need:

A plastic spoon. Preferably a large one, like a dessert spoon. Not a desert spoon, that's something quite different.
Some plasticene, modelling clay, blutack, or similar. Play-dough is too light, sorry.
A plastic ruler. 30cm is fine, but 20cm is better if you can find one.

What you do:

Snap the bowl off the spoon (be careful to aim it away from yourself when you do this, in case it fires splinters), and fill it with a big blob of plasticene. You want it to look like a heaped spoonful of plasticene.
Now imagine you hadn't snapped the handle off, and line the spoon up with the ruler. Slide one over the other until the bowl of the spoon is dead-centre of the ruler. In a moment, you're going to push the two together so the plasticene sticks them, but just before you do, twist the bowl of the spoon. Just turn it, ever so slightly - five or ten degrees is plenty. Now squidge them together
You'll have to juggle everything around a bit so that when you put your rattleback on a table, spoon-side down, it balances nicely. You want the ruler to rest parallel to the table, without one end touching or one side being especially lower than the other. Smudge the spoon around until you've got that. Done? Good
Now, spin your rattleback. Give it a good flick with your fingers so it spins several times, and see what happens. When it stops, try spinning it in the other direction.

What's going on

What you should see is that your rattleback is quite happy spinning in one direction (either clockwise or anticlockwise), but if you spin it the other way, it objects. It'll start to rock, and then rattle, and then - amazingly - it'll stop spinning, and simply rock back and forth like a see-saw. If you're lucky, it'll even start to rotate the other way.

It's around about now that you'll want an explanation. Which is, unfortunately, extremely hard. The hand-waving argument is that the asymmetry of the rattleback is crucial. Remember you skewed the spoon? If you unskew it and spin the thing again, you'll find that it'll go either way quite happily.

If you think about the rattleback rocking back and forth, and the way the skewed spoon twists the ruler, it's fairly easy to see how it might start spinning from the rocking motion. Everything else is about the rattleback transferring energy from one sort of motion (rotation) to another (oscillation - rocking) and back again (rotation in its 'preferred' direction).

The full explanation was only worked out in 1986, by a physics professor from Cambridge. But if you want to look it up, you'll find more about these curious toys on this link.