Picture books have long been a favorite way of introducing mathematical concepts in my class. What’s even better is when you can take one book and apply it across grade levels. Mathematical concepts grow throughout the grades, and perhaps nowhere is this more evident than in geometry, when the concept of shape starts before students even enter school. Marilyn Burns, mathematician and author extraordinaire, captures a simple idea of adding sides to a triangle in The Greedy Triangle but this one picture book can support learning throughout the grades.