I'm trying to make a combotorial proof of this:

$\displaystyle {2 \choose 2} + {3 \choose 2} + {4 \choose 2} + ... + {n-1 \choose 2} = {n \choose 3}$

I've proved this using algebra and induction pretty easily. It was just basically expanding it out and cancelling terms. However, I have no idea how to give a combinatorial proof. I've looked at examples in my text book and they all seem to vary and there's no clear way of how to approach a combotorial proof.

Can anyone help get me started?

Thanks!