There are two people in front of you holding guns.
One is your best friend and the other is an impostor who looks just like him and knows all that he knows.
You also have a gun and must shoot the impostor, if you get it wrong, the impostor will shoot you. If you get it right you and your friend are safe.
What is the one question you can ask that will reveal the impostor?
Ask them to drop the gun. The impostor will not do so.