You come across two people. One is a liar who always lies and the other is a truthteller who always tell the truth. You don't know who is who though.
Both these people are allowed to say one sentence each. What shall each of them say to prove their identity?
Note: Both need to prove their identities individually. Just the truthteller proving his identity or just the liar proving his identity is not enough.
Source: I made this question up myself.