Show that atmosphere exerts pressure.

Explanation:

Put in some little water in a can and boil it until all the water evaporates. While being careful, cover the can with its lid so that it is airtight. Pour very cold water on the can and you'll observe the can crumbles inwards. This is due to atmospheric pressure which is higher on the outside of the can than inside.

Another observation is during siphoning.  Put one end of a pipe inside a tank with water and suck out the air in the pipe on the other end (Remember the end you suck on must be at a lower level than the water level in the tank – this creates a potential difference). You notice that when sucking out all the air in the pipe and the water starts exiting the end, it continues to draw water from the tank with no more sucking. This is also due to atmospheric pressure applying to the water in the tank.

b. a lightning bug, plug of grass, and a worm