>You're a traveler in a distant land which is inhabited by two races. The first race, we'll call them Glurphs, can not ever tell a lie, and the second race, we'll call them Rafas, can't seem to ever tell the truth. Being a stranger to this land you are not familiar with the ways of the people there and when you meet a group of three citizens you ask them which race they belong to.
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>The first one mumbles something that you could not understand.
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>The second one explains "He said he was a Rafa."
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>The third one retorts "You're a liar!"
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>Your problem is to figure out which race the third man belonged to.

If the first man is Glurphs, he would say, he is Glurphs.
If the first man is Rafas, he also would say that he is Glurphs.

So, the second man is lying. And therefore he is Rafas.

And the thrid is obviously telling the truth by saying that he is a liar, so he is Glurphs.