Why is it when you siphon gas out of a tank the liquid keeps flowing?

A siphon works partly on the same principle as a drinking straw: a difference in pressure.

The atmosphere exerts a pressure on everything on Earth.  This is because air is pulled down by gravity in the same way we are.

When you suck on a straw, you decrease the air pressure in the straw and the liquid is pushed up into the straw by the difference in pressure between the inside and outside of the straw.

A siphon can be explained in the same way.

To start the flow of liquid you suck on one end of a tube (just like a straw) and, just like a straw, the liquid is pushed into the tube by the difference in pressure.

Once the tube is filled, the exit end of the tube must be placed at a lower level than the liquid you are trying to siphon.  The liquid in the exit side of the tube is then pulled down by gravity, creating a vacuum in the tube.

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