There are $$2$$ blue marbles and $$2$$ yellow marbles in a bag. To guarantee that Lucy can get $$2$$ marbles of the same colour, she should take at least~\uline{~~~~~~~~~~}~marbles. 
Overseas Competition->Knowledge Point->Counting Modules->Permutations and Combinations->Combinations Overseas Competition->Knowledge Point->Combinatorics->Pigeonhole Principle->Worst Case in Pigeonhole Principle Problems