Transcript:
Truck reads: GASOLINE
Sluggo: Why do gas trucks always have a chain dangling the back?
Nancy: That's to prevent an explosion.
Sluggo: Gotta go to th' store---See ya later.
Nancy: What's that?
Sluggo: A can of fluid for my uncle's lighter.
Without researching… My guess as to how that prevents explosions is that it keeps the truck grounded so it won’t cause static electricity and set the gasoline on fire