The excercise called for filling a glass bottle, without air, and putting it in the freezer. The boys predicted what would happen, and the book proivded us neat information on why. We just didn't predict it would happen with such penache.
We did more than one bottle. All around, it was interesting results.
Dangerous? Surprisingly so! I had Gavin put a dish towel over the bottle in the freezer as the book warned of shards of glass. There was a great deal of popping and cracking, especially when we took it out of the freezer. Neat.