mathematical ideas than the vast majority of us will ever use1. Without these ingredients, the curriculum either does not really involve mathematics, or it is not for all. How to teach a “mathematics for all” becomes clearer when we recognize that mathematics is both a body of facts accumulated over the millennia, and a body of ways of thinking that has allowed people to discover or invent these facts and ideas. In our view, teaching these ways of thinking, which we refer to as mathematical habits of mind, is a vital part of mathematical instruction at every level.In this chapter, we discuss some of the habits of mind that are especially relevant in grades K-5, and offer some concrete suggestions on how they fit into everyday teaching and learning.Some Habits of Mind for the Elementary GradesThe sections that follow describe five habits of mind, their roles outside of mathematics, and how mathematics extends, specializes, or sharpens each of them. Because the language arts curriculum is such an important part of elementary schooling, and because communication is such a good example of a broadly important skill that math can help develop, we focus a lot of attention on communication-related habits of mind. Our hope is to help you step beyond the particular examples we give, so that you can apply the principles throughout your teaching.Habit of Mind 1: Thinking about Word MeaningsCreating, testing, adjusting, and working with definitions is a major part of mathematics. Though children rarely need to create formal definitions, all of the skills of doing so are part of expressing oneself clearly. A definition—especially outside mathematics—is hard to do right, as it must include all cases of the thing to be defined but not allow anything extra. Consider, for example, trying to define chair. Little children might just give it a utility: “You can sit on it.” But pillows, tables, horses, and the floor also fit that description. A category, such as “a piece of furniture,” also doesn’t do, by itself. It may help to combine the two, or to say not just that you can sit on it but that “its primary purpose is for sitting on.” Trying to make definitions helps children (and the rest of us!) see why they can be so difficult to understand. How often have you seen a child look up a word in the dictionary, perhaps even copy the definition faithfully, and then write something like: “At night, we extinguished [put out] the cat.”It is valuable for children to become attuned to the ways words are used, whether for the logical reasoning that mathematics requires or just for the sake of expressing themselves clearly. Mathematics gives many opportunities for thinking about and playing1 This is a tough distinction. Mathematical ways of thinking—even some of the most sophisticated ones—are often used by people who don’t use “mathematics” at all in their work or pleasure, and who consider themselves poor at and ignorant of mathematics. On the other hand, even such a mathematical big deal as the Pythagorean theorem—which is a big deal precisely because it pops up all over the place in mathematics (not just in geometry), in forms that sometimes disguise it quite well—is not something that most of us ever use explicitly unless our business is the doing or teaching of math, science, or engineering