* Fire Hazard: Gasoline fumes are highly flammable. A running engine creates sparks and heat, increasing the risk of ignition. Even a small spark can ignite fumes, leading to a fire or explosion.
* Regulations: Many gas stations have signs prohibiting it, and in some areas, it's actually against the law.
It's much safer to turn off your engine, remove the key, and avoid using your cell phone (another potential spark source) while fueling.
