Try this experiment to see the effect of air pressure.

--> See the flash animation

You will need
a) a glass
b) some water; and
c) a piece of thick card

Steps

1. Fill a glass with water up to the brim then place a piece of card over the top.

2. Keep your hand on the card and turn the glass upside down.

3. Now take your hand away- the air pressing up against the card stops the water falling out of the glass. Air pressure is very powerful even though we cannot see it.