system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: how does the petrol pump know when to stop?
gpt
Suction. 

There is a small hole in the tip of the nozzle that is connected to the handle by a small pipe, when the tank is not full, this suction is pulling in air, and remains open, but when the gas rises enough to block the hole, the suction stops, releasing the handle lever.