White, Black and Brown

Professor Merle White of the mathematics department, Professor Leslie Black of philosophy, and Jean Brown, a young stenographer who worked in the university's office of  admissions, were lunching together.

"Isn't it remarkable," observed the lady, "that our last names are Black, Brown and White and that one of us has black hair, one brown hair and one white."

"It is indeed," replied the person with black hair, "and have you noticed that not one of us has hair that matches his or her name?"

"By golly, you're right!" exclaimed Professor White.

If the lady's hair isn't brown, what is the color of Professor Black's hair?

(Source: My Best Mathematical and Logic Puzzles by Martin Gardner)

The solution to this in the back actually uses a surprising amount of unnecessary reasoning about the genders of the people involved. It is given that no one's hair matches their name, so Professor White doesn't have white hair. Professor White responds to the one with black hair, so, barring talking to oneself, Professor White doesn't have black hair either. This means Professor White has brown hair. Professor Black is then required to have white hair, which solves the problem. (But, for completion, Professor Brown can only have black hair.)