You are in front of two doors. One door leads to incredible wealth and the other leads to nothing of value; unfortunately, you don't know which is which. Two people (who do know which door is which) stand in front of the two doors. One person always tells the truth, the other always lies. You do not know which is which.
You are allowed to ask one Yes/No question to one person to determine which door leads to the riches.
What question do you ask?