What you need:
- two straws
- glass of water, at least half full
What to do:
- Place one straw in the glass of water.
- Suck some water out of the straw – it’s easy, right?
- Put the second straw in your mouth, but don’t put the other end in the water, let in hang down by the side of the glass.
- With the two straws in your mouth, try to suck up the water.
What happens:
You should be unable to get water up through the straw when you have the second straw in your mouth. When you suck on just one straw, you are chaing the air pressure between your mouth and the water, causing the water to move up the straw and into your mouth (the lower pressure). When you stop sucking, the water falls back down the straw into the glass because the pressure is equal again. When the second straw was in your mouth with the other end in the air, you weren’t able to achieve the lower pressure in your mouth because air was coming up the second straw, keeping the pressure the same.