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?
gpt
The gas nozzle contains a tube where the gasoline flows.  There is a second tube within that tube.  This second tube connects to a space filled with air.  As air flows normally through this tube, the air space stays inflated.  When the gas tank gets full, the air tube starts to fill with the gasoline.  This changes the air pressure, causing the air to move out of air sack and push out the gas in the tube.  This action deflates the air sack, which triggers the shut-off mechanism.