There's a small hole near the tip of the pump nozzle. This hole is connected to a small pipe that travels through the nozzle to the handle. When you fill the gas tank, air flows through the hole into the pipe. When the tank is full, the hole will be below the gasoline level and no air will pass through the pipe. A mechanical switch detects when the flow of air stops and shuts off the gas. http://www.sciencetheatre.org/ask_st/122591.html http://auto.howstuffworks.com/question25.htm3.1.2007 05:00Helsingin kaupunginkirjasto