The presence of atmospheric pressure can be demonstrated using the following activity.
1. Take a metallic can and add a little water in it.
2. Remove it's lid or cap and heat the can, as shown in Fig. A
3. The water boils and steam starts coming out from the mouth of the can.
4. The steam forces out most of the air from the can.
5. Now the can is closed with an air tight lid.
6. Now on pouring cold water on the can, we observe that the can crushes.
When the can is heated and then capped, the hot air is filled inside the can.
Due to the cold water, the steam inside the can, condenses to liquid state. Thus, a partial vacuum is created. In this case, the external atmospheric pressure becomes greater than the inside pressure. Hence high external pressure crushes the container.