Drinking Straws and Pop!

This is one of those cool physical tricks that's difficult the first time, but extremely satisfying when you get it right, and extremely annoying for everyone else.

What you do:

You have to do all of the following quickly, as one sequence of events, so it's imperative that you and your accomplice have worked out how you're going to achieve it. Alternatively, you can be bumbling idiots so long as you have enough straws to practice with. Your choice.

1. Hold the straw sideways in front of you, then fold over the ends - just a centimetre of them. Pinch the folds very tightly between your thumbs and forefingers.
2. Now wind the straw up. The action is rather like twiddling your thumbs, only with a straw in the way. You'll find that the ends of the straw wind in towards the middle, which looks increasingly like it's bulging with all the air being squeezed into it. Uncanny, that, since it's bulging with all the air being squeezed into it.
3. Your accomplice comes into play when you have about three centimetres of straw left, bulging away between the wound-up ends. Stop winding,and - quick, quick! - get your accomplice to flick that bit with their finger. They should flick hard, but should aim for the straw and not your knuckles. Especially after they've tried that gag the previous dozen times.
4. If everything goes to plan, you should hear a sharp and very loud 'crack!'

What's going on:

Folding over the ends of the straw very nearly seals them, so when you wind the straw up you squeeze a straw's-worth of air into just that middle part. The pressure in there is very high - if you allow for a little air to leak out, it's still perhaps twice normal atmospheric pressure. You have to finish the trick quickly or the air will find its way out of the ends, which won't be perfectly sealed however tightly you're squeezing.

The flimsy straw can handle the pressure - but only barely, and only because a smooth cylinder is the best shape of pressure vessel there is. Flicking the straw distorts its cylindrical shape, increasing the stress at the sides. The result is that the plastic bursts, and you'll probably see a long slit up the side of the straw.

The loud noise you hear is the compressed air escaping rapidly through the slit.

If you're particularly dextrous and practice a bit, you might be able to flick the straw all on your own, without needing an accomplice at all. It's hard, though - click here to see a video of it being done (2MB .mov file - requires quicktime to play).