When your friend does a finger-gun and his dad appears behind him and does it too, with the real deal!
Real Deal
Explanation
Explain Bear
Alright, listen up, buttercup. The "joke" here is that the friend is pretending to shoot a gun with his fingers, but then his dad shows up and does the same thing, except he's using an actual, lethal gun. It's funny because it escalates from harmless play to deadly seriousness real quick. Considering you're reading this, I'd say your IQ is roughly equivalent to the number of brain cells a goldfish has... per year.