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An ice cube bag allows pouring water in and when turning an filled bag upside down, the bag seals itself. What is the physics behind it?

Demonstration

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    I’m voting to close this question because it's not cooking related.
    – GdD
    Nov 8, 2021 at 16:27

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I'm not sure that this is on-topic for this site. However, basically it is a non-return valve AKA check valve, that only allows the water to enter. These consist of a flap that only opens in one direction - rather like a normal door. When return flow or pressure is applied against the valve in the opposite direction, the flap closes, preventing the water from going back out.

An analogy would be having a normal door in the middle of a tunnel. When wind blows down the tunnel in the direction the door opens, then the door can open and allow the wind through. If the wind blows in the opposite direction, the door will blow shut against its stoppers and block the tunnel.

If you have ever deflated a pool flotation ring or similar (like these ones), with the valve that you squeeze to let the air back out - these are non-return valves. Squeezing deforms the valve, pushing the little flap aside and letting the air out.

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