like blowing up a balloon. The air pushed inside the tire and lifted the truck up.”

Mr. Figueroa turned back to Eduardo. “Is that what you were saying, Eduardo?”

Eduardo nodded.

Billy went next. “I sort of agree with Eduardo that you’re putting more stuff in the ball and so it should get heavier. Like, if you added sand or water, it would definitely get heavier. But I don’t think you can weigh air. It’s like too light, too small. So I think it will still balance, or maybe get lighter. Can I vote for two predictions?” Everyone laughed.

“We have lots of different theories on the table, and they’re all interesting. Does anyone want to agree or disagree with any of these predictions?” More hands went up. One student said, “Just do it!” Then several said, “Yeah, let’s find out.”

“I still want to hear what more of you think,” Mr. Figueroa said. “Let’s go around so everyone gets a chance to explain their predictions.”

The discussion continued for about 10 more minutes, with students arguing for each of the different alternatives.

Finally, Mr. Figueroa said, “Okay, let’s do it and find out.” He walked to the pan balance, which still had the dark-colored volleyball on the right-hand side. As he was about to place the light-colored volleyball on the left pan, he looked back at the students and said, “Has anyone changed their minds? You know, scientists often change their minds after discussing things with other scientists. So, stand up for your prediction one more time. Do you think the yellow volleyball with 15 pumps of air will be heavier and tip to the left, lighter and tip to the right, or stay balanced?”

Once again the students stood up and tilted their bodies, but this time several more voted that the light-colored ball would be heavier. When Mr. Figueroa put it on the pan, it tilted to the left. The students cheered.

“So what have we learned?” Mr. Figueroa asked.

“You can weigh air!“ Marisa said. Then after a pause, she added, “Does that mean that if I take a big breath of air when I get on the scale at the doctor’s I’ll weigh more?”

Weighing volleyballs may seem a long way from the kinds of science experiments students will do in later grades, but it is actually a prototypical scientific investigation. Students are making predictions based on working theories about the way air behaves, using evidence from their own observations and experience (with balloons, tires, sand) to support their positions. They are using their reasoning abilities to draw inferences about something they can’t see. They are organizing the world in very specific ways to test their predictions, and they are taking careful note of the resulting evidence. Then, they attempt to reason about what they have learned and think about other situations in which their new understanding might be relevant.

Behind the volleyball activity are two important ideas that will lead to an understanding of the atomic-molecular structure of matter: air is something, even though you can’t see it, and air has mass and can be weighed. Later, students will learn that air is made up of tiny air molecules that are moving around very quickly. That might cause confusion when thinking about the air in a volleyball being weighed, since the molecules are bouncing around constantly in all directions. However, the molecules that are bouncing side to side balance each other out, so the ball doesn’t move sideways. But the molecules in the ball are being pulled down by gravity, so the ones hitting the bottom of the ball exert more force than the ones hitting the top of the ball. Therefore, when more air is added to the volleyball, more molecules hit the bottom of the ball with more force than before, and this force registers on the scale.

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