Combinatorial proofs of the identity $(a+b)^2 = a^2 +b^2 +2ab$ The question I have is to give a combinatorial proof of the identity $(a+b)^2 = a^2 +b^2 +2ab$.  
I understand the concept of combinatorial proofs but am having some trouble getting started with this problem, any help would be appreciated. 

Hint.  You have $a$ different blue shirts and $b$ different pink shirts.  In how many ways can you choose one shirt to wear today and one to wear tomorrow?
