Two third graders stay behind after school, get past the building's security system, and break into their teacher's supply closet. That is the back half of this book and the store's own copy says so on the product page, so nobody is being ambushed here.
The front half is a group project. Ada, Nina and Milton have skipped the clay and construction paper diorama everybody else built and made a working scale model of a historic silver mine. Then Milton announces he added his own touch to it right before they turned it in, and Ada is certain he has wrecked it.
This is the fourth book in the series, and it is the one with the most actual engineering in it.
The science it actually teaches
Start with the model, because it is easy to walk past.
A scale model is not a craft project. A scale model means every single dimension has been divided by the same number, and holding that same number steady across a whole build is proportional reasoning, which is the idea a great many children hit a wall on in about sixth grade. Here it arrives as a thing three eight year olds are arguing about on a table. Your child meets ratio as a constraint rather than as a worksheet, years before anyone writes it with a colon in it.
Then there is what is being modelled. You cannot build a mine you have not looked up. A shaft has to go somewhere, it has to be held open, and the rock and the people have to come up and go down. A model of a real place forces the research to happen before the glue does, and that is a lesson about building anything.
The security system is the other half, and it is the reason this one sits highest of the four on a science shelf. A security system is a set of sensors, and the whole trick is that each sensor detects exactly one thing and is completely blind to everything else. Getting past one is not magic and it is not luck. It is knowing what that particular device measures, and then not doing that. There are very few chapter books that will hand a seven year old the idea that a machine does not see the world, it measures one narrow slice of it and reports on that slice.
The last piece is the one I like most, and it comes out of the group project. Milton changed a shared build at the last minute and told nobody until afterwards. That is an untracked change to a system three people depend on, and it is the single most common way real engineering goes wrong. Grown adults with expensive job titles cause outages this way every week. Your child gets to watch it happen at eight, with a diorama.
Set this next to the first book in the series. In on the Case, Ada has a claim and goes and tests it. Here she has a suspicion about a classmate and acts on it without checking, and the entire plot is the cost of that. Same shelf, same girl, opposite lesson, and that is a good reason to own both rather than one.
The child it lands with
The planner. The heist-film child who narrates the plan before anyone has agreed to it. The one who wants to know how the beeping thing at the shop door knows, and will not accept a shrug.
It also lands with the child who has been burned by a group project, which by nine or ten is most of them. The frustration in the first half of this book is extremely accurate and it is not treated as a small thing.
One place to stop and talk
Do this one in two parts, because the book has two questions in it.
Stop when the plan for the closet starts coming together and ask your child what the alarm can actually detect. Not what it does. What it measures. Sound, or movement, or a door coming apart from its frame, or a beam of light being interrupted. Then ask what a person would have to do to be uninteresting to that particular device. That is engineering, and a seven year old can do it.
Then, when the book is finished, ask the other question: was any of that all right? Ada was sure Milton had wrecked the model. She was sure, she was not certain, and she acted on it anyway. Ask your child what it would have cost her to go and look first. Then ask whether anybody has ever been that sure about them.
What to know going in
This is the caution on this book and it is not a small one, so I will be straight with you.
Two children deceive their school's security system and break into a locked closet that belongs to an adult. The book plays it as a caper. The tension is whether they get caught, not whether they should be doing it, and the story is on their side the whole way. For a lot of children that is a great adventure and nothing else. For a rule-following seven year old it can be genuinely unsettling, and for a rule-testing nine year old it is an idea you may not want to have handed over without a conversation attached. Either way the conversation is yours to have, not the book's, because the book does not have it.
The other thing sitting under the plot is that Ada decides a classmate has sabotaged her work and acts on that decision without evidence. Nobody is hurt and nothing is frightening. But the moral weight of this book is on suspicion, so if you have a child who has recently been accused of something they did not do, know that going in.
The store band of grades 2 to 5 runs generous at the top like the rest of the series. Treat it as a grade 2 to 4 independent read. And this is the fourth book, so it reads better with at least the first one behind it.
Where to get it
Do not start here. This is book four, it leans on a friendship the first book builds, and a child who meets Ada in the middle of a break-in has met the wrong Ada. Read On the Case first, then come back for Ada Lace and the Impossible Mission at $6.99, which is the one to own if you have a child who plans things out loud before anyone agrees to them. All four are under Stealth Science.
If your child liked this one
Books we have written about that sit close to this one, by reading level and by subject.
Ada Lace, on the Case: a Hunch Is Not a Hypothesis
Ada Lace, on the Case by Emily Calandrelli
Grade 2 to 5
Amazing Magnetism: Science You Can Check With a Magnet
Amazing Magnetism (the Magic School Bus Chapter Book #12) by Rebecca Carmi
Grade 2 to 4
The Giant Germ: How an Infection Actually Works
The Giant Germ (the Magic School Bus Chapter Book #6) by Joanna Cole
Grade 2 to 4
Browse every book we have written about by age, by kind of book and by subject: Find the right book for your child. Or see the whole Stealth Science shelf.
