The old glass of water trick revisited

The original trick

Fill a drinking glass with water.

Cover it with a 3 x 5 index card or other thin stiff paper.

Turn the glass and paper over.

The paper will remain attached to the water glass.

The extension:

To Do and Notice

Use plastic from an overhead transparency instead of paper.

Once the plastic is hanging below the glass, tip the glass to one side until water starts to dribble out the bottom. Notice that as water flows out, air bubbles enter the top edge where the paper and plastic meet.

Empty all of the water out of the glass. Notice that the plastic remains attached to the empty glass.

What's Going On?

When the water runs out the pressure in the glass is reduced. Some air is forced into this low pressure region so you see it bubbling in. However, the air does not return the glass to atmospheric pressure. In the end the low pressure in the glass in combination with atmospheric pressure outside, holds the plastic against the empty glass. A wet rim of water around the edge of the glass where it meets the plastic helps create a seal to keep the air from moving freely between the outside and inside of the glass.

Return to Draft Activities

Scientific Explorations with Paul Doherty

© 2000

1 Feb 2000