After reading the book, The Greedy Triangle by: Marilyn Burns, I had my students complete this geometric mobile that goes up to an octagon. I called it the shape shifter mobile because the triangle visits the shape shifter to get one more side and one more angle. It is important that you have already taught (or your students already have the background knowledge) on 2D shapes- names, sides, and angles. Once the students have finished filling in the blanks on the shapes I allowed them to color the shapes in. Then I punch a hole on the top and bottom of the shape and use yarn or string to connect them.