1. The Role of Air Pressure

• The atmospheric pressure outside the bottle is pushing up on the bottom of the upside-down bottle (which is now on top).

• Inside the bottle, there is no air to push down on the water from the inside (because you filled it completely, eliminating air bubbles).

• This creates a pressure imbalance. The external atmospheric pressure is stronger than the very low pressure inside the bottle.

2. The Role of Water's Properties

• Cohesion: Water molecules are attracted to each other, forming a "skin" at the surface (surface tension). This skin helps hold the water together as one body.

• Adhesion: Water molecules are also attracted to the molecules of the plastic or glass bottle. This creates a slight seal at the rim.

The external air pressure pushing up, combined with the surface tension of the water and its adhesion to the bottle, effectively holds the water in place. The system is in equilibrium.