# Math Help - Combinatorial Proof

1. ## Combinatorial Proof

Provide a combinatorial proof that:

(2n choose n) = Sum from k=0 of n of [n choose k]^2

I have a couple starting points but I'm not sure if they qualify as a combinatorial proof. Any ideas on how to start this?

2. See the end of this subsection of the Wikipedia article about the binomial coefficients.