Usually the best option if you feel you are dealing with a half-wit. Usually not worth the trouble to say ANYTHING.

Sometimes, unfortunately, this would be the MOST polite thing you could do, because anything you could possibly say would be unflattering. Speaking to one of the clients at the vet I work at, she told me how she had two kittens up for adoption, placed at the Petsmart where she volunteers; one of the kittens had one of his ears missing. Someone actually, and SERIOUSLY, asked if they could get a discount because he wasn't a whole cat. If I had to hear someone say that, I would have to pretend to have not heard them; otherwise I would have to yell at them.