One time during a school lunch break, we sneaked into the chemistry lab and liberated the Chem. teacher's storeroom keys. The idea was to have a laugh with some alkali metals, namely Potassium, and nothing else. However, my friend decided it would be a novel idea to shut and seal all the windows, turn on the all desktop gas taps and leave the Potassium in the sink. This crude timer was nothing more than a beaker with a huge lump of the metal in the bottom which was then blu-tacced to the bottom of the sink. The tap was then turned on and we ran. About 5 minutes later we heard an almighty woosh, a sound of breaking glass and quite a bit of shouting.
The force of this blast had put every window in the room out, set fire to the curtains and the roof. To top the lot though the chemical storeroom from which we'd liberated the metal was ablaze and the fire brigade's hazmat team had to be called out which in turn called for the entire school to be closed for two days.
We were never rumbled either!