Measure the Earth with Eratosthenesa question-and-answer game for children
The product
- A browser game where children aged 10 and above work out how Eratosthenes measured the Earth, by answering his questions.
- Play
The problem
- Children learn great discoveries as facts: simplified, with the details left out.
- They miss the leap behind the idea, and the details that had to be worked out.
- I believe working through a discovery would change how children see the world and the problems in it.
- So I built a way for them to work through one discovery themselves.
My role
Defined the core:
- a Socratic, question-and-answer chat
- the stages of the problem for four age groups, with their props and hints
- hand-written exchanges for every stage
Refined it with the agents, playtesting as we built.
Directed the build with two agents that shared one changelog:
- a design agent: UI and UX, copy, dialogue, props, animation, simulated test sessions
- a coding agent: the game, the server, the tests and the simulated test runs
Had the code audited against the design.
The key decisionwrite every reply by hand
The aim: the game is a starting point, not a course. It builds the intuition to connect what a child already knows to a problem that seems unrelated.
The issue: a language model writing replies from a script revealed answers and invented questions.
The decision: given the narrow scope, I wrote every reply by hand.
- Code settles clear answers ("shadow", "50").
- A small model (Claude Haiku) judges the rest against 11 fixed verdicts.
- Two low-cost models screen messages for prompt injection and off-topic requests.
Quality first: quality set the design. Cost stayed low because a model is called only where code can't decide.
The trade-off: replies vary less, with only minor variations.
What shipped

A 3D world with a chat, and stages set by the child's age.
Interactive props let children test shapes and angles and see what changes.
Hints that get more direct. Each wrong guess brings a more direct hint. The last is too obvious to miss. If it is missed anyway, Eratosthenes gives the answer and moves on.
Recorded conversations. Every conversation is recorded for review, to improve the game.
What I'd do next
Test with children. No child has used it yet; it has been tested only with simulated users. Next is a classroom playtest.
Add parental consent before storing emails or chats (India's DPDP Act).
Test on Safari and on phones.
Move to a paid Groq plan before traffic passes the free tier's 30 requests a minute.













































































































