The Setup: A bottle is filled to the absolute brim with a liquid, ensuring no air bubbles are trapped inside. The cap is screwed on tightly, creating a seal.
The Inversion: When you turn the bottle upside down, gravity tries to pull the liquid down and out.
The Pressure Imbalance:
- Inside the bottle: A near-perfect vacuum (a space with very low pressure) forms where the cap is as the liquid tries to fall. There is no air inside to push the liquid out.
- Outside the bottle: The weight of the Earth's atmosphere (atmospheric pressure) is pushing on every surface of the bottle. This means it is pushing upwards on the bottom of the inverted bottle with a significant force (~14.7 psi at sea level).
The Result: The external atmospheric pressure pushing up is greater than the force of gravity and the low pressure inside pulling the liquid down. This pressure difference holds the liquid in place. The liquid may stretch slightly, and a tiny bubble might form, but the flow is effectively stopped.

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