Smullyan/Black_hat
Three logicians, A, B, and C, are wearing hats, which they know are either
black or white but not all white. A can see the hats of B and C; B can see
the hats of A and C; C is blind. Each is asked in turn if they know the color
of their own hat. The answers are:
A: "No."
B: "No."
C: "Yes."
What color is C's hat and how does she know?