if n items are put into m containers, with n > m, then at least one container must contain more than one item. - Dirichlet’s box principle

This seemingly obvious statement, a type of counting argument, can be used to demonstrate possibly unexpected results. For example, given that the population of London is greater than the maximum number of hairs that can be present on a human’s head, then the pigeonhole principle requires that there must be at least two people in London who have the same number of hairs on their heads.

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