You are standing in front of two gates (a left one and a right one) – one leads to Heaven and the other leads to Hell. You don’t know which gate leads where. Beside the gates, there are two angels. One of them always tells the truth and the other always lies, but you don’t know which one is which.
You have one question to ask one of the angels, in order to find out which gate will lead you to Heaven. What would that question be?
The question is: “What would the other angel say, if I asked which gate leads to Heaven?”. And do exactly the opposite of what the angel says.
If the left gate leads to Heaven:
- The angel that tells the truth will say that it’s the right gate since it knows that the other angel will lie.
- The angel that lies will also say that it’s the right gate since it knows that the other angel will tell the truth.
The same explanation applies if the right gate leads to Heaven.