One day I found my daughter Samantha drinking her juice in a very unusual way, the straw was not even in the cup of juice. I thought to myself, how is this possible and of course I found the answer in science. Keep watching to learn exactly how this trick works and how you can try it yourself at home.
Materials to Collect
- Clear straw
- Glass of water
- Thin plastic or oversized playing card that covers the mouth of the glass
- Bowl to catch any spills
- Put your straw into the water.
- Cover the top of the straw with your finger.
- Lift the straw out of the water.
- Cover the top of the glas with the rigid plastic or playing card.
- While holding the plastic in place, tip the glass upside down.
- Release the plastic and be amazed as the liquid does not spill out!
What is the Science?
The air around us exerts pressure on us. At sea level, it pushes on everything at about 14.7 pounds per square inch. The reason we don’t feel it is because it pushes equally on all sides. The straw has solid sides, so the air can only affect the liquid in it through the openings in the top and bottom. When you pick up a straw from a drink, the air rushes into the top opening of the straw and pushes the liquid down into your cup. But if you put your finger over the top opening, air can longer push on the liquid from the top, leaving only the bottom for air to push on. Water molecules also like to “stick” together, this is called surface tension. (Ever see a liquid in a cup form a dome over the top of a glass?) Between the air pushing on the liquid and the water molecules sticking together, the water will not fall out of the bottom of the straw. The same thing is happening with the upside down glass. Air pressure cannot push down on the water because the solid glass blocks it. The plastic covering the opening is being pushed up by the air pressure from below, holding it in place. This air pressure is heavier than the water in the glass and keeps the cover on and the water in the cup.
Ask Your Young Scientists
While you are doing this experiment, ask your scientist:
- What happens when you release your finger from the straw while there is liquid in it?
- With water in the straw and your finger over one end, tip the straw so the opening is pointing up. What happens to the water? Why do you think this happens?
- How big of a glass can you get to be upside down with water in it? (provided you have a big enough cover.)
More to Explore
Some other things to try:
- Try putting your finger over the straw BEFORE you put it in the water. What do you notice?
- Try different width straws. Does the size make a difference?
- How big of a glass can you get to be upside down?
- Try different types of containers. Find one with a narrow opening and compare to one with a larger opening. Do you see any difference?
- Try placing an upside down cup in a bowl of water. What do you notice happening inside the cup? Does the water get inside?
Check out some of these activities from the Connecticut Science Center!
Find more air pressure experiments here!
We want to see what you try at home. Share your experiments with us on social media by using the #ScienceAtPlay and tagging @CTScienceCenter.
Andrew Fotta is a STEM educator at the Connecticut Science Center. He has currently holds a CT teaching certification for grades K-6, and has spent time in the classroom in nearly all grades, and taught middle school science. In addition to teaching classes for the Science Center, Andrew is also part of a team of educators currently creating new programs aligned with the new Next Generation Science Standards for grades PreK-9. Andrew is an avid photographer, who enjoys blending science and art in his work.