In this demonstration, students will use differences in air pressure to force an egg into a bottle.
Air expands when it is heated. If air is heated in a container that prevents expansion, the pressure of the gas increases. When the heat source is removed, the air will contract and decrease in pressure. In a container, this causes the outside air particles to push into the container to equalize the pressure inside and out. Air always flows from a high-pressure system to a low-pressure system ("air flows from high to low").
In this demonstration, the lit matches heat the air inside the bottle. When air is heated it expands and some of it escapes out the bottle. When the matches go out, the air inside the bottle cools and contracts (takes up less space), thus creating a lower air pressure area inside the bottle than outside. Normally, the higher-pressure air outside the bottle would come rushing in to equalize the lower-pressure air in the bottle. The problem is that the egg is in the way. The air molecules on the outside of the bottle push the egg into the bottle.
Teacher Note: The air pressure does not decrease due to the burning of the oxygen inside the bottle. It is due to the air expanding and contracting. If students believe 'using up oxygen' is what causes the egg to be sucked into the bottle challenge them to design an experiment to prove or disprove this theory.