Air occupies space
To visualize that air occupies space.
A common demo for this is performed by keeping paper dry in water. This demo also demonstrate the the same principle but in a different way.
transparent glass, jar, water bottle with a cap
Fill the jar three-fourth with water and float a bottle cap over it. Guess what will happen if you push an inverted glass over the floating cap. Will the cap go down? Will the glass push the water down?
Hold the glass with its mouth over the cap. Push straight down into the water. The cap is also pushed down to the bottom of the jar. What prevents water from entering the glass?
Move a step ahead. Take a transparent plastic glass and make a small hole at its bottom. Repeat the above activity with this glass. This time why does water enters into the glass?
When a hole was made in the glass, a connection was established between the air outside the glass and the inside the glass. Hence, as water comes in, the air escapes out through the hole.