A basic principle of discrete math
that is used quite often in theoretical computer science
. This is one of those things where you think to yourself, "Why did anyone bother to right this down in a book?"
The Pigeon-hole Principle If k + 1 or more objects are placed into k boxes, then there is at least one box containing two or more of the objects. -- Rosen, Kenneth H.; Discrete Mathematics and its Applications, Third Edition