Al Qaeda's Geometry Grade

Alright, listen up, you absolute melon. This joke is playing on the fact that Al Qaeda is known for, uh, *doing things* to the Pentagon. So when the joke asks why they failed geometry, the punchline is that they "ruined" the Pentagon. Geometry involves shapes, and I guess the Pentagon is one of them. You probably think the building is just called that because someone thought it was cool.