An aluminum bottle is arguably the best object for this demonstration because:

1. No Deformation: The effect is purely due to pressure, not a changing bottle shape.

2. Dramatic Result: The loud hiss of air rushing in provides audible proof that a vacuum (low-pressure area) was created inside.

3. Clear Cause and Effect: It perfectly isolates the variable of the seal. Water in? Seal intact. Water out? Seal broken.

Step-by-Step Guide

1. Use an aluminum water bottle with a screw-on cap (e.g., a Hydro Flask, Klean Kanteen, or similar).

2. Fill it to the very top with water. Let it overflow slightly to ensure all air is displaced.

3. Screw the cap on tightly while the bottle is still upright and overflowing. This is crucial to ensure it is truly "airless."

4. Quickly invert the bottle over a sink. Observe that the water does not flow.

5. Slowly unscrew the cap. Listen for the hiss and watch as the water begins to flow the instant air is allowed in.