Why do we turn it off

So why do we turn off the car when we put gas into it.

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Zaphod42 · 51-55, M

If the gas fumes in the air hit a stoichiometric ratio of between 7% and 15% it becomes explosive, and the spark plugs in your engine could ignite it.

It would take a perfectly windless day for that to happen, but better safe than sorry.