Explain with the help of an activity that air exert pressure.

A very easy experiment to show that air exerts pressure:

Experiment: Take a plastic bottle ( the thinnest plastic that you can find). Suck the air out of the bottle. You can do this by using a vaccum pump.

Observation: The bottle distorts in shape as the air is being sucked out the bottle.

Conclusion: The air outside the bottle exerts pressure on the bottle because of which the shape of the bottle is altered.