For me, it was a 2 liter soda bottle, ~1 square foot of tinfoil, and muratic acid. fold the the tinfoil and slide in the neck of the bottle, add ~2 inches of acid, cap tightly, and throw as far away as you can, and avoid the giant white cloud of gas when it finally blows.
There was some lesson to be learned after that, but I don't rightly remember what it was. (it was an allegory or something- this was a sunday school lesson at the church I used to attend.)