What You Need:
- Piece of card or postcard
- Food coloring (this will make it easier to see the experiment, but is optional)
What You Do:
- Have your child put water in the jug and add some food coloring, if desired.
- Ask your child to pour the water into the glass all the way to the top.
- Put the card on the glass and holding your hand over it quickly turn the glass upside down.
- Now for the fun part: Remove your hand. The card will look as though it is "holding" the water in the glass!
- Get your child to try the trick herself! What does she think has happened?
What's Going On?
The card is not the force that is keeping the water in the glass. It is the pressures on the card from the water and the air on the other side of it!
The pressure of the water in the glass on the piece of card is much less than the pressure of air below the piece of card. This keeps the postcard firmly pressed on the glass so that no water can flow out.
Ask your child what she thinks would happen if she repeated the experiment with an empty glass. What about a glass that is half full of water? Ask your child to try it out and to explain the science behind her results.