Everything kids need to know about triangles—vocabulary, explanations and all.

Beginning with the definition of a triangle and a breakdown of its parts—sides, angles, vertices—Adler quickly launches into a discussion of angles, even teaching kids how they are named, measured and classified. A clever activity instructs readers to cut out a triangle, any triangle. By tearing off the corners and lining them up so the vertices touch, kids can see that the angles of a triangle always sum 180 degrees. Vocabulary is printed in bold type and defined within the text, each new term building on the ones that have come before: “All three angles in ?ABC…are acute angles. ?ABC is an **acute triangle**.” In the primary-colored digital illustrations, a dark-skinned boy and a light-skinned girl are accompanied by a robot as they progress through the book, drawing and studying triangles. They accumulate materials to make another robot and then identify the angles and triangles that make up its body. Labels and diagrams make the learning easy, while the endpapers show several examples of each type of triangle presented. A final activity challenges readers to use their arms and hands to find and name angles—a turn of the page supplies labeled answers.

With lots of layers of information, this is a book that can grow with kids; new information will be accessible with each repeat reading.