Why No Flood?
Until you learn how to work this experiment perfectly (and maybe even then), it's best to work it over a sink or basin.
What you need:
a piece of cardboard or a large index card
What to do:
Place the cardboard over a drinking glass filled to the brim with water. Make sure no air bubbles enter the glass as you hold the cardboard against it. Then turn the glass upside down over a sink or basin. Take away the hand holding the cardboard.
The cardboard stays in place and the water stays in the glass.
The pressure of the air outside the glass is greater than the pressure of the water inside. It is the air pressure that keeps the water in the glass.