Mechanics🎨
Why Every Color Appears Exactly Twice (And Why It Matters)
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Enough reading — go solve something
Every idea on this page is one tap away from practice. The first world is free, no download, no sign-up.
▶ Play Flow PuzzleLoading…
Every idea on this page is one tap away from practice. The first world is free, no download, no sign-up.
▶ Play Flow PuzzleWhen I was building the level generator, the rule I kept coming back to wasn't about colors or grids or difficulty. It was this: every color appears exactly twice. Two endpoints, one path between them. It sounds like a cosmetic choice. It's actually the engine the entire puzzle runs on, and once you understand why, you'll never look at a board the same way.
Think about what "exactly twice" guarantees. Each color has a clear start and a clear end — no ambiguity about what connects to what. There's no third red dot to wonder about, no dangling endpoint. A color is a question with exactly one shape of answer: a single unbroken line from this dot to that dot.
If colors could appear three times, the puzzle would become a branching network, and "connect them" would stop having one obvious meaning. The two-endpoint rule is what keeps a flow puzzle feeling clean instead of soupy. It's the reason you can glance at a board and immediately know the question, even when the answer takes minutes to find.
Here's the part most players never consciously notice. Because lines can't cross and every cell must be filled, the two-endpoint rule quietly forces a beautiful piece of math: the puzzle is a way of partitioning the entire grid into colored paths. Every single square belongs to exactly one path. No overlaps, no gaps.
That's why a flow puzzle is genuinely a logic puzzle and not just a connect-the-dots tracing exercise. You're not asked to link dots — you're asked to slice the whole board into snakes, each one starting and ending on its matching pair. The endpoints are just the labels telling you where each snake's head and tail must be.
Add up the lengths of all the colored paths and you always get the total number of cells on the board. On a 7×7 that's 49. If your paths cover 47, two cells are empty and you haven't solved it — no matter how pretty the lines look. Coverage is the real win condition.
A puzzle is only satisfying if there's a real solution and you can reason your way to it. The two-endpoint structure lets our generator prove a board is solvable before it ever reaches you. We start from a fully partitioned grid — paths already covering every cell — and work backward to hide them, leaving only the endpoints. By construction, a valid solution exists. You're never staring at an impossible board wondering if you've been cheated.
I wrote more about that whole backward-generation process, and the thousands of boards that didn't survive it, in the level design retrospective.
Knowing the theory is nice. Here's how it changes your moves:
The best game rules are the ones that look trivial and turn out to be load-bearing. "Each color twice" is the whole skeleton of Flow Puzzle wearing a very simple coat. It's why a 5×5 in the Mr. Bean world and a 12×12 in The Hero world are recognizably the same game despite feeling worlds apart — and a big part of why scaling the grid changes everything while the soul of the puzzle stays exactly the same.
Next time you open a board, take half a second to appreciate it: two of each color, one grid, one tidy way to carve it up. Then go carve.