A deep container
Paper or paper towel
Fill the deep container with water.
Stuff some paper or paper towel into the bottom of the glass. Ensure that the paper fits perfectly and does not slip out.
Flip the glass upside-down and carefully push it into the water.
Make sure you hold the glass straight. Even if the glass is tilted a little, you will notice bubbles which can further lead to water entering the glass.
When the glass is kept straight, the paper in it will remain completely dry.
How does it work?
Air takes up space when the glass is submerged in water. The air gets trapped between the water and the glass. As long as the glass isn’t tilted, the air remains trapped inside, keeping the paper dry.
When the glass is tilted, the air escapes, creating bubbles and allowing the water to enter into the glass. The water rushes in and takes the place of air, thereby making the paper wet.