Designing a one-rule number puzzle: one path, every tile, target sums in order

Sumthread is a number puzzle with exactly one rule, and we spent most of the design time making sure that rule never needed a second paragraph.

The rule

Trace one continuous path through every tile on the grid. Up, down, left or right. No diagonals, no revisiting a tile. Along the way, your running sum has to hit the target sums shown at the top, in order.

When your running total reaches the active target, that tile locks with an amber checkpoint ring and the count starts again from zero. The last target has to land on the very last tile of the board.

That's the whole game. It sits somewhere between a Hamiltonian path puzzle and a running-total arithmetic drill, which is why it works as a five-minute break: it's spatial and numeric at the same time.

Design choices that made it feel calm

• Reach cues instead of error messages. Dashed borders show which neighbouring tiles you can step to next, and a warm amber marker shows a neighbour that would complete the current target exactly. You learn the rule by seeing it, not by failing it.
• Backtracking is free. Drag backward along your path or tap an earlier tile to rewind. No penalty, no life counter.
• A mastery goal that isn't a timer. Finish a board with zero hints and zero undos and you earn the Clean Weave seal. Speed is never scored.
• A daily board. One fresh 4x4 Daily Thread a day, with local completion history and a spoiler-free emoji grid you can share.
• Progression by grid shape. 3x3 Starter Threads and 3x4 Morning Break boards, then 4x4 Deep Weave and Quiet Sunday, then 4x5 Grand Tapestry.
• Themes and accessibility. Warm Ivory, Charcoal Dark, Newsprint Cream, Deep Forest, Clay & Ochre, plus optional step-order badges and high-contrast tile borders.

It saves automatically mid-path, plays fully offline, needs no account and has no third-party tracking.

Sumthread: Number Path Puzzle is free on the iPhone App Store: https://apps.apple.com/us/app/sumthread-number-path-puzzle/id6829816144

If you design puzzles too: what's your favourite trick for teaching a rule without a tutorial screen?