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 not a knave.
Who is a knight, who is a liar, and who is a knave?
A can't be a knight.
If B is a knight, then A is a knave, leaving C to be the liar, but that means he'd be telling the truth. So B is not the knight.
So C is the knight, and either A or B is the liar. But if B is the liar then his statement is a true onea contradictionso A is the liar and B is the knave, telling a lie at this point.

Posted by Charlie
on 20120521 17:56:47 