Why do gasoline tankers usually have metal chains at the back?

Gasoline tankers often have metal chains at the back to prevent the buildup of static electricity during loading and unloading operations. As the tanker moves and the gasoline flows through pipes, friction can generate static charges, which pose a risk of ignition if they discharge. The chains help to ground the tanker, allowing static electricity to dissipate safely into the earth, thereby reducing the risk of fire and explosion.