It is apparent that if there are 6 apples in 5 drawers, at least one of the drawers contains 2 apples. This simple yet powerful principle is often stated as the "pigeonhole principle". It is one of the fundamental tools in combinatorics. To know more about it, you may visit http://db.math.ust.hk/notes_download/elementary/combinatorics/de_D4/e_control.htm or one of the winning essays http://db.math.ust.hk/competition/essay/0405/articles/J3.pdf of the MD essay competition.