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A glass is partly full of water and we hold a cardboard firmly against its top as we invert it. Why does the water stay in the glass and the cardboard doesn’t fall down? The answer is related to **air pressure**.
Air pressure is pushing up on the cardboard from below, while the water is pushing down on the cardboard from above. The **cardboard transfers the force of the air pressure upward to the water**. There is a pressure of **one atmosphere** pushing up on the water from below.
Of course, there is also pressure from the air inside the glass pushing down on the water from above. When we inverted the glass and let go of the cardboard, the water moved downward, thereby **increasing the volume allotted to the air**. As a result, the **air pressure goes down**. The pressure inside the glass pushing down is not as great as the outside pressure pushing up, and this pressure difference is enough to counteract the gravitational force pulling down on the water.
