There are people and strange monkeys on this island, and you can not
tell who is who (Edit: until you understand what they said - see
below). They speak either only the truth or only lies.
Who are the following two guys? A: B is a lying monkey. I am human. B: A is telling the truth.
A: B is a lying monkey. I am human. B: A is telling the truth.
If we assume B is telling the truth then A is also telling the truth. But A in statement says B is lying which contradicts our assumption.
Hence, B must be lying. If B is lying then statement of B suggest that A is also must be lying.That mean 'B is lying monkey' part of statement made by A is too lie. Since we have deduced already B is lying, the lying statement of A suggests that B is not monkey. Hence B is the lying man!
Other lie spoken by A is that it is human.Hence A must be a lying monkey!