Answer:
Air takes up space. It cannot get easier than this! The pressure exerted by the air inside the glasses allows the air to exclude the water and the ping pong ball. Using the tissue just helps to show that air, not water, is trapped inside the glass.
Step-by-step explanation:
The reason this works is that air is taking up space in the bottle. Water cannot enter the bottle unless some air is able to leave the bottle. A liquid can flow and take the shape of its container. Gases don't have a fixed shape or a fixed volume. The particles move around all the time and spread out. Remember, Anything with mass can take up space without you realizing it: Just like molecules can't be seen but are the building material of an object.