Three months after finishing Toy Cars, Carolyn's class began a new matter and energy unit. As part of the initial class discussion on energy, Carolyn asked the class to remind her about how they got toy cars to move.

After a student (Calvin) observed that electric things use batteries, Carolyn asked the class how the batteries make the car go. Hannah thought that some kind of liquid source of energy (which Adrian identifies as acid) in the battery makes the car move. She later drew an analogy between battery acid and gasoline that fuels regular cars. So Carolyn asked, if gasoline and acid both provide energy, but they're different, what's energy? Alec responded that energy gives the car power, while Janet identified energy as a power source that makes objects do what they are supposed to do.

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In the ensuing discussion, students recognized that cars (and other objects, like people) needed to get energy from a source, and they recognized several different forms of energy. In their experiments with toy cars, the energy of the cars came from a variety of different sources.

Students then began a lively discussion about the rubber bands that are part of the mechanism used to make propeller cars move. Carolyn asked whether a rubber band is energy. Though students answered yes, Carolina described a process in which the rubber band gets energy when it's twisted, and then when you let go, the propeller moves. Alec, with assistance from Carolina, elaborated by describing how the rubber band, the propeller, and the air (or wind) gain, lose, and "give" energy to each other and the toy car. He finished by adding "us" to the list of things that have the energy to make the car go. Jamillah then described how energy is tranferred from "us" to the rubber band, and then from the rubber band to the toy car.

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