When I was in seventh grade, my math teacher showed me two separate lines that had arrows at their ends. One had the arrows pointed inward like this: >-<. The other had the arrows pointed outward like this: <->. My teacher asked me which of the two lines was longer and I answered that it was the one with the arrows pointed inward. He took out a ruler and measured them, showing me that they were identical in length! I took another look and—despite my teacher having just proved that they were the same length—stubbornly clung to the belief that…