By Marilyn Burns
A triangle gets bored of being a triangle and asks a shapeshifter for just one more side, and then just one more.
The triangle has a good life. It holds up bridges, it props open sandwiches, it gets to be the slice of pie. But it starts to feel dull, so it visits the local shapeshifter and asks for one more side and one more angle. As a quadrilateral it can be a television, a picture frame, a book. That's fun for a while. So it goes back. Pentagon, hexagon, on and on, and each new shape opens new jobs and then stops being enough. Eventually it has so many sides it can't tell which way is up, and it rolls straight down a hill.
Be clear-eyed about what this is. Marilyn Burns is a math teacher first, and the shapeshifter is a device for getting through the polygons in order. The story is a coat the lesson is wearing, and it fits well enough, but nobody will love this the way they love a folktale. What it does brilliantly is the thing it was built to do: it fixes the names, it ties every shape to real objects your child can then find all over the house, and the ending gives them a reason those names matter. If you want a read-aloud that teaches geometry cleanly in ten minutes, this is the one. If you want a story, pick a different book on this shelf.