> How does the piston going down allow air to be pulled in?
Air near the surface of Earth is under pressure due to the weight of all the air above it being pulled down by gravity. This means that if space opens up it is pushed in by that pressure.
Think about if you have a big tub of water and open up a bottle full of air at the bottom of the tub. The air will be pushed out and replaced by water, right? This basic idea works for air as well except in this case there is *nothing* filling that space the piston opens up.
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