this experiment to see the effect of air pressure.
See the flash animation
a) a glass
b) some water; and
c) a piece of thick card
Fill a glass with water up to the brim then place a piece of card
over the top.
Keep your hand on the card and turn the glass upside down.
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.