If you fill a glass with water but put cardboard on top and turn it over the water will often stay inside the glass and be held up by air pressure.
The cardboard will stay in place due to the air pressure pushing up on the cardboard being greater than the weight of the water that is pushing it down.
Although it will also be dependent on factors such as the size as well as shape of the glass and the type of cardboard being used as well as the precision of the seal.
The atmosphere exerts pressure on everything, including the water in the glass as well as the cardboard.
And when you turn the glass over, a good seal between the cardboard and the glass rim is also crucial.
Also the water molecules have a natural attraction to each other and create surface tension.
This can then help hold the water together and also assist the cardboard in staying in place.
Having a perfect seal is ideal, as even a small gap can allow air to enter and break the balance and cause the water to spill out.
And using a sturdy piece of cardboard which does not easily bend or deform is also crucial for maintaining the seal.
And if the glass is not completely full, there might be enough air to cause the water to spill out.