The core physics principle remains the same: Atmospheric Pressure.

1. Creating the "Airless" Condition: You fill the bottle to the absolute brim with water and seal it with its cap. This eliminates almost all the air inside, creating a very low-pressure environment.

2. The Pressure Imbalance:

   • Outside the bottle: High atmospheric pressure (~14.7 psi) is pushing up on the entire bottom surface of the upside-down bottle.

   • Inside the bottle: There is no significant air pressure pushing down on the water column.

   • Result: This immense pressure difference holds the water in place against the force of gravity. The water may pull down by a millimeter, creating a near-perfect vacuum at the top (now the bottom), but the external pressure prevents it from going any further.

3. The Critical Difference: Rigid Walls
   Unlike a plastic bottle, aluminum is rigid and will not flex or crumple inward.

   • The strength of the aluminum walls resists the crushing force of the atmosphere.

   • This means the vacuum space inside remains stable, and the seal is maintained perfectly. There is no "giving way" like with plastic, so the experiment is often even more stable.