A Golf driver drives a Manta driver at the traffic lights at the back.

A Golf driver drives a Manta driver
at the traffic lights at the back. They both get out and take a look. Says the Golf driver: ?Watch out, you have a great double-pipe exhaust there. You blow into it really hard and then the dent bulges on its own.? Says the Manta driver: ?Good idea, hey.? The Golf driver continues driving and the Manta driver blows full pipe into his exhaust. Of course nothing bulges! A second Manta driver comes and asks what's going on. The first one tells him the whole thing and says: "Man, you're stupid. Nothing can bulge, you still have the windows open."