Cut a fairly thin slice of lemon, and float it in some water in a cereal bowl.  Push some matches into it, with the heads pointing upwards.  (If you can, angle the matches so the heads are close to one another.  It will make things easier.)

Now light the match heads.  Be careful!  Quickly upturn a glass over the lemon.  After a few seconds, the matches will go out – and the lemon and the water level will rise up inside the glass.

The flaming matches burn off the oxygen inside the glass, reducing the air volume.  The water level rises in compensation, and the lemon floats up with it.