system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do gas pumps know when your tank is full and when to stop fueling?

How does the gas pump stop automatically? I know they can't use like any type of electric sensor..can they?
gpt
The nozzle is actually 2 hoses, 1 for fuel that's put into the tank, and a second one for returning air/vapors in your car's tank to the gas station's tanks. 

As fuel fills up your tank, the venturi action of the air up the other hose holds the valve on the nozzle open. When the tank is full, the venturi action stops, and the valve shuts.