Making a paper lid
Warning! Only under adult supervision
- tinted water;
- paper or cardboard.
Fill a glass with tinted water and cover it with a piece of paper or cardboard. Turn the glass upside down, holding the paper lid. Take your hand away from the lid – and the lid stays in place!
The paper lid is held in place by atmospheric pressure, which acts on all objects within the Earth's atmosphere. When the glass is inverted, gravitational forces pull the water in the glass downward. This increases the volume of the remaining air in the glass, and the air pressure inside the glass becomes lower than atmospheric pressure. Moreover, even the total pressure of the water and remaining air combined on the paper lid is less than atmospheric pressure, and the lid holds! It's worth noting that the water’s surface tension also helps hold the paper in place.
Even more cool experiments await you in the MEL Physics subscription!