An acrylic cylinder is filled with water and an inflated balloon is placed over the opening at its top. Both the cylinder and balloon are turned upside down so that the balloon prevents the water from pouring out of the cylinder.
What happens if one lets go of the balloon?
After some drops of water have dripped out of the cylinder, the balloon closes the opening of the cylinder and keeps the water inside.
How does this work?
When a bit of water has leaked out of the cylinder, the pressure inside the cylinder is lower than the air pressure outside the cylinder. Now the outside pressure presses the balloon firmly over the opening of the column and seals it tightly.
In theory, the outside pressure is strong enough to keep a 10-meter water column in a cylinder.
