Good Riddles

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?

Question: What is the strongest bird?

Question: What sort of animal is a slug?

Question: How do you make seven an even number?

Question: There were 2 trains heading the opposite directions on the same track.1 was going North, and one was going south, There was a 100m gap between them, avoiding the collision, 2 men saved the trains from colliding on time, Who were they and how did they save them?