Water Upside Down - Results

Answers

Answer to question 1. During and after turning the glass upside down, the card remains attached to the glass of water. Once you have the glass of water upside down, the water remains in the drinking glass.
Answer to question 2. Any answer is correct, as long as you wrote down what you thought would happen. Most of the time you won't notice that air presses on all things, from all sides. While you are holding the glass upside down, the air presses on the card, upwards. The water inside the glass presses on the card by it's own weight, downwards. The water won't fall out of the glass because the air pressure is stronger than the force of the water.
Answer to question 3. A bigger glass would probably work as good as a small one. The air pressure could even hold a bucket full of water. To make it actually work, you just have to make sure there is no air and no opening between the card and the water.

Extra Explanation

The air is pressing on the postcard as it presses on all things, at the rate of 1 kilo per square centimeter (1 kg/cm2). One liter of water is approximately one kilo. Per square centimeter of the surface of the postcard, the air pressure can hold a liter of water. The drinking glass has a surface of at least 15 square centimeters, so it will be easy for the air pressure to hold the water on the other side of the card.

The postcard is needed because water is thin and fluid. For instance, crème fraîche is thicker and less fluid. As a result, it is easy to hold a cup of crème fraîche upside down without a postcard at all. In physics the thickness of a fluid is called viscosity. Crème fraîche has a large viscosity, so there is more friction between the molecules than in water. Water has a small viscosity so the water molecules will flow more easily. The postcard is there to hold them together.