Defying Gravity

We all know how gravity works at a basic level.  If I fill a glass of water and tip the glass over, the water falls out and spills all over.  However, there is a way to prevent the water from spilling without adding a sealed lid to the cup.  Fill a glass of water, take a note card that is wider than the opening of the cup and place it over the cup.  Now slowly tip the cup upside down.  Guess what?  The water doesn't spill out!

How does this work?  Did we magically defy gravity?  That would be cool, but no, we didn't defy gravity.  There is still a gravitational force acting on the water to pull it down.  However, the force from air pressure pushing back up on the note card is greater than the gravitational force downward.  Thus the water stays in the cup!  Super cool!  Try it out yourself.  Your kids will be amazed.  Try different types of paper.  Does notebook paper work?  Why or why not?