10 years and up.

Pairs of students are challenged to build a slow car using a motorized-LEGO kit. Students begin with a bicycle-building activity which introduces the concepts of gearing down and gear ratios. They utilize this knowledge to design a car capable of low speeds. The cars are timed on a course to see how slow they can go, and students are encouraged to modify and alter their design throughout the lesson.

See this article from the Journal of Research in Science Teaching: Reasoning about structure and function: Children’s conceptions of gears