Correlations vs. Causation.

A while ago, I said I'd write a post with my favourite "things which are correlated". Well, now I've finally figured out how to do page-breaks, I guess it makes sense to write it now. Here are my favourites. Any suggestions as to why they might be correlated? (answers below the fold)

1. Countries which put Fluoride in the water have much higher cancer rates than countries which don't.
2. Schoolchildren's shoe size is very strongly correlated with their performance on spelling tests.
3. The number of shark attacks on a given day is strongly correlated with ice cream sales.
4. You are much more likely to wake up dehydrated if you sleep on a sofa than if you sleep on a bed.
5. The number of prostitutes working in a city is positively correlated with the number of policemen.

The answers are all fairly straightforward when you see them, but the first one, at least, has stumped many people I've shown it to:

1.  Countries that put fluoride in their water are rich! It's only rich countries which can afford to put fluoride in the water, and in which people live long enough to get cancer.
2. Older children have bigger feet, and spell better.
3. Shark attacks tend to happen when people go to the beach.
4. I don't know about you, but I'm much less likely to sleep on a sofa sober than drunk.
5. Bigger cities have more prostitutes, and more police officers (there are lots of variations on this: dogs and fire hydrants... no doubt others).

Anyone got any others?