Third-grader Aven Green has been solving mysteries for a really long time--a whole month! She's solved many important cases like The Mystery of the Cranky Mom, The Mystery of the Missing Ice Cream, and The Mystery of the Smelly Feet. Her record is nearly 100% (only The Mystery of the Cereal in My Underpants remains unsolved to this day). Aven asks all the right questions, wields her detective kit carefully, and follows up on every clue. Then her teacher's lunch bag (with her lunch still in it) is taken and Aven's great-grandma's beloved dog goes missing! Can this perceptive detective crack two cases at the same time? Luckily, Aven has a super-powered brain full of lots of extra brain cells to take on both cases. See, she was born without arms, so all of the cells that were supposed to make her arms went into making her brain instead. At least that's her working theory for The Mystery of Why I Have So Many Extra Brain Cells.