Galileo proposed a geometric model to explain how much force is required to break a horizontal beam that is secured at one end and pushed down at the other—a cantilever.  Preservice teachers at LSU investigated and tested Galileo’s model using styrofoam beams.  They found that Galileo’s model predicts a strength that is 3 times as great as actually observed.  In fact, Galileo’s error was known in the 17th century, and was corrected by Marriotte and Parent by taking elasticity into account.  Their model explains the 3-fold error observed by the teachers exactly.