If it Rains in Summer, Why is a Duck?

I once had a college class (polysci, I think) where the prof put a bonus question on the final which was "If it rains in summer, why is a duck?" He said anyone who answered that correctly would get an A in the class. No one did, (no one got an A either!) and over the years I've become obsessed with the answer to that riddle.
--Vanessa

Vanessa, the clue to the answer of this riddle is contained in your question. No one got an A in your professor's class.