Place a peeled, hard-boiled egg on top of a bottle or jar with an opening slightly smaller than the diameter of the egg. Cool the bottle by setting it in an ice bath. The egg will be sucked into the bottle.
When a gas is cooled, the pressure it exerts decreases. If the pressure inside the bottle gets low enough, the higher air pressure air on the outside will push the egg into the bottle. The same effect can also be caused by heating the air inside the bottle, either by heating it directly from the outside or by dropping a piece of burning paper into the bottle before setting the egg on top, but the reason it works is slightly different.
When the air inside the bottle is heated, it expands. The heated air forces its way past the egg, causing the egg to jump a little on the mouth of the bottle before settling into place and sealing the bottle again. When the bottle is allowed to cool, either because it is removed from the heat or because the paper burns itself out, there will be less air inside the bottle than when the experiment started. The higher air pressure on the outside will push the egg into the bottle.