You have to use a shelled, hard-boiled egg for this trick. Try to get an egg which is free from cracks.
Find a bottle with a neck just small enough that the egg won''t fall in (a milk bottle, for example).
Screw up a small piece of paper and drop it in the bottle. Alternatively, use two or three wooden matches. Light the paper or matches, and allow it to burn out.
Imediately place the egg in to the bottleneck.
The egg will be sucked in, making a very interesting sound in the process!
To our minds, the egg is sucked in the bottle - but in fact, the egg is pushed in. The fire inside the bottle heats up the air, forcing some air out of the bottle and making the air inside the bottle less dense. After placing the egg on the bottleneck, the air begins to cool down again, and the air pressure outside forces the egg into the bottle.