FIGURE 4-5 Mr. Sohmer’s wall-on-wheels.

the wall, and—by accident—I leaned against it, what would happen to the wall?” (See Figure 4-5.)

“It’ll move over there—it’s gonna move to the right!”

“True. And it’s going to keep on moving to the right until—remember, these are frictionless wheels—until it bounces off the end of the room, and comes back the other way.”

Then Mr. Sohmer told the story of the Air Puppies.

“Imagine that Air Puppies represent air molecules. Think about how newborn puppies bumble around constantly, mindlessly, with no intentions at all. They move around constantly, in every direction, like air molecules, without thinking, wanting, planning, or choosing to do anything.”

“Do Air Puppies breathe air like real puppies?” one of the students asked.

Mr. Sohmer responded by introducing a discussion about models and how they are never exactly the same as the thing they represent. Students volunteered examples: Model airplanes don’t fly. Maps don’t include the potholes that are on some roads. A menu doesn’t taste like the food it describes.

“Different models highlight different things,” he explained. “They’re useful in different ways. They make some things visible and other things invisible.”

This kind of discussion about the advantages and limitations of different models helped the students understand how scientific knowledge is constructed and how central models are in the construction of that knowledge. The Air Puppies are the bumbling (mindless) agents in a modifiable drama with a particular setting (always including two rooms separated by a moveable wall-on-wheels). The necessary result of the Air Puppies’ incessant, unintentional bumbling is a completely understandable, completely predictable, and thoroughly lawful effect—that is, the wall moves as it must, given the Air Puppies’ opposing impacts on both sides.

Mr. Sohmer continued the Air Puppies story. In his first version, the two rooms on either side of the wall-on-wheels each contain an equal number of identical Air Puppies mindlessly bumbling around and bumping into the walls and each other. The wall-on-wheels moves whenever a puppy bumps into it (see Figure 4-6).

FIGURE 4-6 The view from above at the beginning of the Air Puppies story showing an equal number and kind of Air Puppies on each side of the wall.

“So what will happen to the wall?”

“It’ll stay in the same place,” a number of students called out. With the aid of a QuickTime movie of an interactive physics animation, Mr. Sohmer demonstrated how the scenario in Figure 4-7 was set in motion. The wall stayed in approximately the same place, oscillating about the centerline (Figure 4-7).

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