It's Mr. Tortoise's birthday, and he can't wait to eat cake with his friends. But there's a hitch! Mr. Tortoise can't remember how old he is, so his friends don't know how many candles to put on the cake. And they won't stop (or slice) until they figure out the mystery.

Could Mr. Tortoise be as many years old as there are sections on his shell? He's twice as big as the smaller tortoise . . . so is he twice as old? After the partygoers work through a variety of possibilities, they find their way to the answer with a simple bit of addition.

With text from I Don't Want to Be a Frog author Dev Petty and hilarious graphic novel-style illustrations from Ruth Chan, How Old Is Mr. Tortoise? will have readers guessing the answer from beginning to end.