Water Barometer - Bottle Collapse
To demonstrate the pressure in a water column.
Two 2-liter soda bottles filled with water connected by tubing.
Two plastic bottles are connected by a plastic tube and the entire system is mostly filled with water. Holding either of the two bottles up in the air causes that bottle to collapse.
Russell Akridge, Water Barometer, TPT 31, 110-111 (1993).