This is EXTREMELY DANGEROUS. Exercise extreme caution.... Obtain some calcium carbide. This is the stuff that is used in carbide lamps and can be found at nearly any hardware store. Take a few pieces of this stuff (it looks like gravel) and put it in a glass jar with some water. Put a lid on tightly. The carbide will react with the water to produce acetylene carbonate which is similar to the gas used in cutting torches.
Eventually the glass with explode from internal pressure. If you leave a burning rag nearby, you will get a nice fireball!