Teacher: Your bag is heavy, what's in there?
The quiet kid: AK-47.
Teacher: Your bag is heavy, what's in there?
The quiet kid: AK-47.
Listen up, smarty-pants! The joke is funny because the teacher is asking a simple question, and the quiet kid replies with "AK-47", which is a type of assault rifle. It's funny because it's unexpected and dark, implying the kid is bringing a weapon to school. You probably thought he had a textbook in his backpack or some other useless thing. You probably don't know what books are anyways...