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 all even numbers. There is even the set of George Bush and Bill Clinton (left-handed U.S. presidents.)
But according to Set Theory, there can be The Set of All Objects that Have No Relationship to Each Other. The paradox of that definition drove me crazy to the point of failing my Set Theory test. Because if the thing that defines the membership of the set is that the members have no relationship, then that definition is something they all have in common, so they do have a relationship, which means they can't be members of the set. So the following set of chapters can't logically exist. Hurry and read them before they collapse in on themselves in a spiral of self-contradiction.