with the way words are used. Think about the meanings of the words circle and on in these two contexts:A definition creates a category, and categories may be related. Many mathematics curricula and high-stakes tests expect elementary school children to know the hierarchy of categories polygon quadrilateral parallelogram rectangle square2, as in thisproblem adapted from a teacher-made fourth grade assessment test.Three of these words are frankly of little use to most people outside of school, but there are two worthy ideas that one can learn from exercises like this. One is that there are hierarchies of categories. This is hard for kids, both in and out of math. Faced with six toy cows and four toy horses, young children regularly trip over questions like “Which is more, the number of cows or the number of animals?” The problem is not with counting. Asked independently how many animals there are, they’ll get the right answer. For very young kids, calling a set of creatures “cows” seems, temporarily, to exclude those creatures from other categories. Does this mean that young children need specific lessons on quadrilaterals to rectify this problem? No! They’ll eventually begin to understand multiple levels of classification anyway. But experience at a developmentally appropriate level helps.A second useful idea is that the way categories are used depends on the context.When we ask students to mark all the rectangles on an exam, we expect them to include the squares. But when we ask students to draw a rectangle, we may well think they’ve misunderstood, or have even been a bit obstreperous, if they draw a square! It is not the category “rectangle” that has changed from the exam context to the drawing context, but the way we use it. The exam item checks whether students understand that the general category “rectangle” includes the special case “square.” The request to draw a rectangle—whether in math class or in casual conversation—calls for a picture that2 The sign means “includes.” Like the > sign, it opens up to the “larger,” more inclusive category. So, things that we’d call polygons (roughly speaking, closed shapes that have only straight sides) include all the things we’d call quadrilaterals (four-sided polygons), and so on. At this point, English begins to be a bit confusing. It is correct, in English, to say “Squares are rectangles,” but “are” clearly does not mean “=.” A comparable statement outside of mathematics is “Bears are animals”: animals bears.