Making gunpowder (I was crafty enough to buy the ingredients from three separate chemist shops) and blowing up a derelict outside privy. I found the formula in my great uncle's scientific encyclopaedia. There was also a page on nitroglycerin but the old boy got wise to my activities and the book disappeared before I could try that one. We made 'soap box' trolleys. Pretending to be a knight astride his charger, I witlessly drove mine through a bus queue, scooping up an old lady on the way. Fortunately I was wearing my medieval helmet (big brycreem tin with a plume of dried grasses) because she clouted me round the head with her rolled up umbrella. One of the workers at a firm next to my primary school was convicted (and hanged) for murdering two children. Before he was caught we used to play football with him during the lunch break. This was in the 'safe' 1950s. Captain Flint in Swallows & Amazons had a brass signal cannon. I recently bought one from a marine antiques dealer. I realise that this thread is about daft things we did before we grew up. I obviously didn't.