Fill a drinking glass with water until it is almost completely full.
Tell your audience that you will turn the glass of water upside down without spilling the water.
Take any glossy playing card or any other card with a glossy finish and press it on the top of the glass. Make sure that the entire top of the glass is completely covered with the card and that there are no gaps between the card and the rim of the glass.
Hold the glass in both hands in preparation to tip it over.
Tip the glass over quickly while holding the card in place at the rim of the glass.
Let go of the card. The card will stay up against the rim of the glass underneath the water. If you have done this trick correctly, the water will not spill.