When my son was in junior high, he had a girl who liked him. While I don’t know all the details, apparently something went wrong and there were some hard feelings. A couple of years later the girl wrote him a letter trying to tell him off a little and maybe make him regret losing out. At one point, she wrote: “Now who’s the dumb??” 

Dumb isn’t, of course, a noun, so it sounded, well… dumb, and as such, the wording of the question kind of provided its own answer. 

We still laugh about this now and again and will quote it when we want to be a little silly.