I'm having a hard time with this problem, could someone point me in the right direction?

How can I show the following is false?

For all sets A and B, and all functions f : A \rightarrow B, g : B \rightarrow A, if g \circ f = I_{A}, then f \circ g = I_{B}.

Obviously, I need to find a counterexample, but I'm having a hard time coming up with one.