You meet three people, A, B, and C. One is a knight, one is a liar, and one is a knave.

A:I am a knave.
B:A is a knave.
C:I am a knave.

Who is a knight, who is a liar, and who is a knave?

The key is that either a liar or a knave can say "I am a knave" but a knight cannot. 

Therefore B is the knight and correctly identifies A as the knave, and so C is the liar.

Posted by Jer on 2013-10-22 13:49:12