A pocket of air is formed in a hen's egg when the contents of the egg contract upon cooling after the egg is laid. This "air cell" is formed as an inner membrane separates from an outer membrane, and the air pocket is generally located at the larger end of the asymmetric egg. A close look at the shell of an egg reveals thousands^ of tiny pores through which carbon dioxide and moisture in the egg may exit over time, allowing air to enter. As a consequence, the air pocket developed during laying increases in size as the egg ages. During the boiling of an egg, the increase in temperature causes the air pocket to increase in volume. If the boiling occurs too rapidly, the expanded air volume doesn't have time to diffuse through the porous shell, causing the egg shell to crack. The enlargement of the air sac over time has two interesting consequences for the consumer: eggs that float in water and hard-boiled eggs with variable ease of peeling. The phenomenon of an egg floating in water is a consequence of an air cell that has enlarged sufficiently over time to keep the egg buoyant. The ease of peeling a hard-boiled egg is also related to the age of the egg. The fresher the egg, the smaller the air sac. The older the raw egg, the larger the air sac and more the egg contents must contract to accommodate the enlarged air sac. A greater degree of contraction leads to an easier-to-peel shell.