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When a glass partly filled with water is inverted with a piece of cardboard held firmly against its top, the water remains in the glass, and the cardboard does not fall. This phenomenon is due to air pressure. Air pressure beneath the cardboard exerts an upward force, while the weight of the water pushes down from above. The cardboard transfers the upward force of the air pressure to the water, balancing the system. The upward pressure exerted on the water from the air is equal to one atmosphere. Inside the glass, there is also air pressure pushing down on the water. When the glass is inverted and the cardboard is released, the water moves downward, increasing the volume available to the air trapped inside the glass. This causes the air pressure inside the glass to decrease. As a result, the outside air pressure pushing upward is greater than the reduced pressure inside, creating a pressure difference that counteracts the gravitational force acting on the water. This keeps the cardboard in place and prevents the water from spilling.
