When I was in elementary school, they told us that what are called the nine planets were really eight planets and a rogue moon. Then I went to high school, and they told us that there were most certainly nine planets. Then I got to college, and took an intro course where they told us there were somewhere between eight and hundreds of planets in this particular solar system.
This morning on the TV news the anchors were agonizing over what the revelation that Pluto may not be a planet will do to the school children who’ve worked so hard at rote memorization of the planets. This should suggest something about the limits of rote memorization as an educational strategy. And it offers a useful reminder to students and teachers that teachable moments are sometimes the ones that cast doubt on what you’ve been taught.