Once an egg is laid, its yolk and albumen (egg white) cool and, as a result, contract. This shrinkage creates an air pocket at the egg's larger (nontapered) end. The air in the pocket can cause the shell to crack when you place the egg in simmering water (or, if the cook insists, in boiling water). The reason is that the heat of the water expands the trapped gas (air) in the pocket, creating an atmospheric pressure many times greater than exists in either the water or your kitchen. This built-up pressure easily cracks the fragile shell, releasing the trapped gas and, unfortunately, allowing some of the albumen to ooze into the water.

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