A Physics teacher used to enjoy tricking his students into learning science facts. One day, he placed an 8-ounce glass filled with water on a table. He then produced 2 straws; one straw he placed into the glass of water and the other he held outside the glass.
“Class,” he said, “I am willing to fund a pizza party for the class if any one of you can drink this glass of water in 5 minutes using only these two straws.” The class was amazed; this would be an easy task for any of them. “There are a few conditions,” he continued, “You must drink using both straws at the same time. You must drink using slow and steady draws. You cannot block or obstruct either straw in any way. You cannot lift or tilt the glass off of the table. And finally, one straw must remain outside of the glass.”
The class selected their champion and felt confident that they would be enjoying pizza at the same time next week. Five minutes later, the champion returned to his seat, defeated, and the class sat with jaws agape. Being a good sport, the teacher still bought pizza for the class the next week, confident they would never forget the lesson he taught them.
Why did the champion fail?
The teacher knew a lot about air pressure. He knew that air from a high-pressure area would always seek to fill an area of lower pressure, such as a vacuum. He also knew that a straw worked on this principle. Drawing on the straw creates a low-pressure area in the mouth; air pressure pushes down on the water and forces it up into the straw. When he placed the second straw outside the glass, and thus outside the water, he gave the air a more direct path from the high-pressure area to the low-pressure area. With slow steady draws on the straws, the air will easily rush in through the second straw to fill the low-pressure area, and the water will remain in the glass. Try it at home!