It’s fun to drink with a straw but did you ever wonder exactly why a straw works? Straws work because of something you can’t see, but is everywhere. Sound like magic? Nope—it’s science!

Even though a glass may look empty before you fill it with your favorite cold drink, it’s actually quite full—full of gas that is. We are surrounded by the atmosphere, which is a good thing, since it contains the oxygen we need to breathe. All of that atmosphere is pushing down on us (don’t worry—we have gas inside our bodies that is pushing back, so we don’t get crushed).

What does this have to do with a straw? Well, think about what happens when you put a straw into liquid. The liquid rises in the straw, to exactly the same height as the liquid in the glass. That’s because the atmosphere is pushing down on the liquid in the glass. The atmosphere pushes the liquid in the glass up into the straw. Since the atmosphere is also pushing down on the liquid in the straw, the liquid rises, just until the force of the atmosphere on the liquid inside the straw exactly balances the force of the atmosphere inside the glass. That happens when the levels are even.

So, why can you use a straw to get the liquid up into your mouth, higher than the level in the glass? When you inhale through the straw, you take the atmosphere out of the straw (it goes into your lungs). Now the atmosphere is pushing down on the liquid in the glass, with noting the balance it inside the straw, so the liquid rises…into your mouth!

Since we can’t see the atmosphere, it’s hard to figure out how it works. Scientists deal with this problem all the time. Designing experiments to measure things that can’t be seen is a big part of a scientist’s job. Here are a few experiments you can try to see the atmosphere!

Seeing the Atmosphere

• Straw
• Drinking glass/drinkable liquid
• Plastic/paper cup
• Index card that covers the entire cup.
• A tissue
• A tub or sink to work in.

1) Take a straw. Drink something from it to be sure that it works. Now, make a small hole in it (make sure the hole is large enough to see). Make sure the hole is above the level of the liquid in the glass. Does the straw still work? What happens if you cover the hole with your finger and then try to drink? When you suck on a straw that has a hole, you pull air through the hole, instead of removing air from the straw. The liquid stays put, unless you cover the hole.

2) Fill a small plastic/paper cup with water. Put the index card on top, making sure that the card completely covers the cup. Put the palm of your hand on top of the card (to hold it on) and turn the cup over. When the cup is completely upside down, take your hand away. What happens to the card? To the water? (Do this over a tub or sink, just in case!) The card remains attached to the cup. The atmosphere is pushing up with more force than the water pushes down, so the card stays attached.

3) Take a tissue. Crumple it up and place it in the bottom of a cup. If the tissue won’t stay in the bottom of the cup, use a small piece of tape to secure it. You want the tissue to be squeezed into the bottom of the cup. Turn the cup over and push it into the water. If you are careful to keep the cup level while pushing, the tissue will stay dry. Why does this happen? Even though the cup looks empty, it’s full of air. When you turn the cup over and push it into the water, the air in the cup pushes away the water and the tissue stays dry!