• ✓Idea: the adding is the easy half — holding a lookup table while doing something else is the hard half
• ✓The key never leaves the screen. This is not a memory test.
• ✓Read-the-key mode: nine squares, no arithmetic yet
• ✓Picture sums: look up twice before you add once
• ✓Colour path: the numbers name colours, tap the dots in order
• ✓No repeats and no gaps in the key, so a question can never have two answers
• ✓3 misses lights the key entries the question needs — the move, not the answer
• ✓6 misses breathes, then straight back — no question can be failed
• ✓A subtract mode, and three-picture sumsplanned
• ✓A vegetables pool — only carrot and corn are painted so farplanned

✓ = done · ☐ = planned. Every game is built one step at a time like this.