Over the centuries, as philosophers, scholars, and scientists built upon their colleagues’ ideas to explain
motion, their theories about the world emerged and changed. Both evolutionary and revolutionary
metaphors of this process offer insights. Looking back over two millennia, we can see original ideas
emerge, flourish, or falter as a result of the intellectual climate, and broader theories gradually evolve. On
the other hand, each insight and change in worldview was undoubtedly a revolution in thinking for the
individuals who moved this process forward. These two metaphors—evolution and revolution—may
prove helpful to teachers and curriculum developers in designing experiences to help students develop
their own useful theories about gravity. We return to these ideas in our concluding remarks.