Unleaded, Please

A woman at a gas station noticed a spaceship landing in front of her. An alien stepped out of the spaceship and started to pump gas into it. The woman noticed the letters ''U.F.O.'' printed on the side of the ship. She turned to the alien and asked ''Does U.F.O. stand for Unidentified Flying Object?''
The alien answered, ''No, it stands for Unleaded Fuel Only!''