What you need 
- Glass
- Piece of cardboard
- Water
- Sink or tub
How you do it
- Completely fill a glass with water.
- Place a piece of cardboard over the top of the glass.
- Turn the glass upside down over a sink or tub, holding the cardboard tight against the rim of the glass.
- Now remove your hand. Ta-daaaa! The water should not spill out! (If it does, practice a few more times over a sink. Make sure there are no air bubbles inside the water glass)
The Science
Do you know why the water stays in the glass? It has something to do with pressure. The water stays in the glass because the pressure of the water inside the glass is less than the pressure of the air that pushes from the outside against the cardboard.