Chris and I bought the book (by Sean Connolly) for the boys: 64 daring experiments for young scientists. We finally got around to doing the Cola Geyser (first experiment in the book) because we don't usually have a 2 liter of Diet Coke (corn syrup in regular Coke ruins it) and mentos just laying around. It was awesome and we're totally going to have to do this again.
1. Slide mentos into freshly opened 2 liter.