Twin Tumblers

Light a candle stump in an empty tumbler, lay a sheet of damp blotting paper over the top and invert a second tumbler of the same size over it. After several seconds the flame goes out and the tumblers stick together.

During combustion the oxygen in both tumblers is used up - the blotting paper is permeable to air. Therefore the pressure inside is reduced and the air pressure outside pushes the tumblers together.