For some reason they decided we needed to learn Set Theory in the fourth grade. It drove me nuts. Not that the rules of Set Theory were hard to memorize. Intersection and Union, big deal. What bothered me was the definition of a set—there wasn't any definition! A set could be anything, even nothing (remember the null set?). In my fourth grade mind (which hasn't changed all that much yet), a set was supposed to have something in it, and those things were supposed to be related to each other somehow. A salt and pepper shaker set. The set of...