The aforementioned chemistry teacher had a nice little demonstration using an empty syrup tin with a small hole in the top and bottom and a bunsen burner.
Fill the tin with methane from the bunsen, then turn off the gas tap and light the escaping gas at the top hole. At first it burns slowly, like a small candle flame, then as the gas/air mixture reaches the explosive ratio the lid flies off with a satisfying bang. The day he put the lid on a bit too tight was very memorable, but the lab got new ceiling tiles and he was asked not to do it again!