-cup (in my opinion a clear glass works the best).
Step 2: Cardboard
Cut the cardboard so it is a little bigger than the glass.
Step 3: Water
Fill the glass about 1/4 of the way full. Then place the cardboard on the top of the glass.
Step 4: Done!
To start out put your hand on the cardboard and slowly tilt the glass upside down so that all of the water is sitting on the cardboard. Then slowly and carefully remove your hand off the cardboard. the cardboard should stay on the glass! Any sudden movements will cause the cardboard's seal to fail resulting in a mess full of water.