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(More items than containers)
If you put n+1 items into n containers, at least one container has more than one item. This is the pigeonhole principle.
Example: You have 13 people. At least two were born in the same month. Why? 13 people, 12 months. Pigeonhole principle.
This principle proves existence. It doesn't tell you which container has duplicates, just that one must.
