Explain with the help of activity …

Take a tin can with a lid. Fill it with water about half of it. Heat the can till the water boils. Blow out the heating device like candle. Put the lid tightly on the can. Put the can carefully in a washbasin. Pour fresh water over the can. You see that the shape of the can gets distorted. As water is poured over the can, some steam in the can condenses into water, reducing the amount of air inside. The pressure of air inside the can decreases and becomes less than the pressure exerted by the air from outside the can. As a result the can gets compressed. This activity confirms that air exerts pressure.