Begin by demonstrating how the car works. Explain how the propeller can pull air around it's blades when it is spinning. As you wind up the rubber band, explain how energy is being stored in it that will be used to turn the propeller.
Briefly explain what inertia is. Basically, it is how much energy an object needs to begin moving. It's important to keep the car lightweight to have little inertia so the propeller doesn't have to work as hard to get the car going.
Next, I walk the students through how to build each component. I make one set of wheels in front of the students and just describe the other steps. Before allowing the students to begin building, I tell them to feel free to try new ideas and experiment :)
By building a propeller-powered car, students will comprehend three basic elements of a wheel assembly: the axle; the bushing; and the wheels. Students will also improve their fine motor skills by using a hot glue gun.
By operating a propeller-powered car, students will experientially comprehend: how energy can be stored; how to use potential energy to turn a propeller; and how the turning propeller creates propulsion.
As always, please be sure to talk about glue gun safety with your students if they have never used glue guns before. I've found that kids as young as 7 years old can safely and confidently use a hot glue gun if given proper instruction and guidance.