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 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 curring torches.  Eventually the glass will explode from
internal pressure.  If you leave  a burning rag nearby, you will get a nice
fireball.