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Gasping at Straws

This experiment comes from a question sent in by a list member who wanted to know how a drinking straw works. At first this seems to be a very simple thing, but like most very simple things, the more you try to explain it, the more complicated it gets. To explore this subject, you will need:

- a glass of water
- several drinking straws
- a pin or needle

First, lets try using the straw in the usual way. Feel free to substitute other liquids for the water. I used cold orange juice and found it delightful. Place the straw into the liquid and drink a few swallows through the straw. As you drink, pay attention to exactly what is happening. What are you doing to cause the liquid to rise up the straw into your mouth?

To help you see what is happening, lets try it again, with one difference. This time, use two straws. Place both straws in the liquid and try drinking through both at once. It still works very well. Two straws work just about as well as one. Now lets make another change. Leave one straw in the liquid and hold the other one beside it, but on the outside of the glass. With one straw in the liquid and the other in the air, try drinking with both straws. Does it work as well this way?

No. In fact, it does not work at all. The only thing you get in your mouth is air. Why? To understand the straw, we have to understand air pressure. The air around you is pressing in on you with tremendous force. At sea level (where I live), the air presses in on you with 14.7 pounds of pressure on each square inch of your skin. It also presses on the glass and on the water in the glass.

While the straw is just sitting in the glass. The air pressure is the same inside and out. When you drink through the straw, you seal your lips against the straw and then you use your lungs and mouth to reduce the air pressure in your mouth. You do this by expanding your mouth or lungs. There is now more space to contain the same amount of air, so the air pressure is less.

At this point, the air pressure in your mouth is lower than the air pressure surrounding you. Your mouth is connected to the straw, so the air pressure inside the straw is lowered as well. There is the same amount of pressure on the outside, but there is less pressure on the inside. The outside pressure is pushing the water up the straw harder than the inside pressure is pushing it down, so the liquid moves upwards.

When you used the two straws, one in and one out of the liquid, the straw on the outside of the glass allowed air to enter, equalizing the air pressure. Since inside and outside are pushing equally, the liquid stays where it is. You can see this in a different way by using a single straw and making a tiny hole in it with a pin or needle. This also allows air to enter, preventing the straw from working properly. If you make the hole very small, someone might try using the straw without realizing why it did not work, but of course you would never do that, would you?