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?
If A is a Knave, then all three statements are true, which can't happen if one is a Liar. A cannot be a Knight, so A must be a Liar. This means B's statement is false, so B must be a Knave. This leaves C as the Knight.