Examples to show that air exerts pressure:

1. When air is pumped inside a balloon, it expands in size. This shows that the air inside exerts pressure on the walls of the balloon. 

2. A sealed packet of chips swells up in the mountains. This is because the atmospheric pressure at higher altitudes is less whereas the packet is filled with gas at normal atmospheric pressure. It therefore swells up in the mountains, as the pressure inside is more than the atmospheric pressure.