I put a paper plate on top of a glass of water. I turned it over, and the water stayed in the glass.

The weight of the water is pushing down on the paper plate, but the plate stays in the glass because the pull of gravity is being balanced by another force. What is that force?

1. Attraction

   No. The slight attraction between the water and the glass is not enough to balance the pull of gravity.

2. Air pressure

   Yes! Because the plate is keeping outside air from entering the glass, outside air pressure is keeping the plate in place. As long as the outside air pressure is enough to balance the weight of the water and the plate, it will stay in place. If you made a small hole in the glass to let outside air get in, that would unbalance things, and the water would fall out.

3. Surface tension

   No. The water tension at the surface of the water would not balance the force of gravity.

4. The weight of the paper card

   No. Gravity is pulling down on the paper plate and the water. The weight of the paper does not help balance the force of gravity.