Amaze your friends and family with your ability to turn a glass of water upside down over their heads without the water gushing out... A glass Water Piece of thin card – a postcard or a playing card will do A towel in case it all goes wrong! 1. Half fill the glass with water. 2. Place the card over the top of the glass making sure that it is completely covered. 3. Holding onto the card, turn the glass upside down. 4. Slowly remove your hand from the card. 5. Hey presto – the water remains in the glass! It might be wise to practise this a few times over a sink before you try it out on your granny. It is also wise not to hold the glass upside down for too long – once the card gets too soggy, the "magic" runs out and the water will give someone an unwelcome shower! As with all tricks, you’ve probably realised by now that there isn’t any magic happening here. The water stays in the glass because of a surprisingly powerful thing called air pressure. Although we can’t see it, the air around us is exerting a pressure in all directions. When you turn the glass upside down, the pressure of the air pushing up against the card is greater than the effect of gravity trying to pull the water out of the glass and so the water stays put. When the card gets soggy it becomes flexible and deforms making it difficult for the air pressure to hold the card onto the glass. Eventually, though, the card will bend so much that the water can escape from the glass.