What You Need & How to Do It

1. Get a plastic water bottle with a cap. A glass bottle works too, but plastic is safer for demonstration.

2. Fill it to the absolute brim with water. No air bubbles should be left at the top.

3. Screw the cap on tightly while the bottle is still upright and full.

4. Quickly turn the bottle upside down. Do this over a sink or basin at first, until you're confident.

Observation: The water will not flow out. You might see a few drops fall or a small air bubble form at the top (now the bottom), but the main volume of water will stay in the bottle.

Why a Tiny Air Bubble Matters

If you see a small air bubble appear at the neck of the upside-down bottle, it's because the water might have stretched slightly downward, creating a tiny low-pressure space. A tiny amount of air then forces its way in to equalize that very small pressure difference. Once that tiny bubble is in, pressure equalizes and the flow stops again.