In mathematics, the pigeonhole principle states that if
items are put into
containers, with
, then at least one container must contain more than one item. This theorem is exemplified in real life by truisms like "in any group of three gloves there must be at least two left gloves or at least two right gloves". It is an example of a counting argument. This seemingly obvious statement can be used to demonstrate possibly unexpected results; for example, that there are two people in London who have the same number of hairs on their heads.