There are $$4$$ black socks, $$4$$ grey socks and $$4$$ red socks in a drawer.  Cindy is supposed to get her father a pair of socks but the light in the room goes off.  How many socks must she get from the drawer so that there is definitely at least one pair of the same colour? 
Overseas Competition->Knowledge Point->Counting Modules->Permutations and Combinations->Combinations Overseas Competition->Knowledge Point->Combinatorics->Pigeonhole Principle->Worst Case in Pigeonhole Principle Problems