At school, in the 5th year we had a mate called Peter Herold. He was ever so clever - we were all in the top class but he was streets ahead of the rest of us. One day in Chemistry we were working with sodium metal. Sodium metal is kept in oil because it gets extremely hot and reacts in a very dramatic explosive way when it comes into contact with water. It's also very soft and you can cut it with a craft knife.
Peter Herold cut off a piece and wrapped it in one of these green lab paper towels and placed it in his pocket. As the day wore on it started to heat up in his pocket - to the point where it got rather too hot and he decided to dispose of it. He threw it down the toilet. The toilet exploded. Shards of porcelain and gallons of water everywhere. He very nearly got expelled for that :)