Good Riddles

 
Choose One (medium)

Question: There're two people in front of you: your friend and an imposter looking exactly like your friend. There's no way to determine who is who by asking questions, they know the same information about you. You have only one bullet in your gun. If you kill or injure the imposter, you and your friend will be saved, If you're wrong, he'll kill you immediately. How will you know?