[Schubfachprinzip][1] ("drawer principle" or "shelf principle" or  "Dirichlet's box principle"). It is now easy to guess we are talking about [P-H][2] [P][3].


  [1]: http://mathoverflow.net/questions/1270/alternatives-to-pi-day/1273#1273
  [2]: http://mathoverflow.net/questions/1270/alternatives-to-pi-day/1273#1273
  [3]: http://mathoverflow.net/questions/4279/interesting-applications-of-the-pigeon-hole-principle