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. 

Combinatorially argue that ${a+b \choose 2} = {a \choose 2} + {b \choose 2} + ab$
