Each multiplication is a flower in a 10 × 10 garden. Practising it waters it: new flowers need water often, strong ones less and less. Spaced repetition becomes something a child understands and wants to look after. A bee, Pipa, guides and reads aloud.
The review engine is FSRS-6, checked against the reference implementation, and lives in a Swift package with no UI. The web version is a port of that engine and uses the same texts and tricks, exported from Swift.
The garden is symmetric: 3 × 7 and 7 × 3 grow together. A flower only turns gold when the memory lasts for weeks and the answer comes without thinking.
Lessons learned
Safari on iOS 12 can't decode the 24 kHz AAC voices, and won't play an `<audio>` element if the host doesn't support range requests. On the web the voices are 44.1 kHz MP3 files downloaded in full and played with Web Audio.