🌈 Flow Fill
Puzzle · Free online · No download · No sign-up
Flow Fill places coloured dots at various positions on a grid. Your task is to draw a continuous path connecting each matching pair of dots — without any two paths crossing — and to fill every cell of the grid with colour in the process. Meeting the first condition is achievable by trial and error; meeting both conditions simultaneously requires genuine spatial reasoning. The grid grows and the dot count increases level by level, turning what starts as a relaxing puzzle into a demanding exercise in constraint satisfaction.
How to play Flow Fill
- Tap a coloured dot and drag to draw a path towards its matching dot of the same colour.
- Release when the path connects to the matching dot; the path locks in place.
- Repeat for each colour pair on the grid.
- If paths cross or leave cells empty, the puzzle is not yet solved — adjust paths as needed.
- Tap an existing path to erase it and redraw from either endpoint.
- The puzzle is complete when all pairs are connected and every cell is filled with colour.
Tips to beat your high score
- Work corners and edges first. Dots near corners have fewer possible route options. Resolving them early prevents you from painting yourself into a corner later when interior routes crowd together.
- Longest paths first. The pair that needs to travel the furthest will naturally claim the most cells. Route it first, then fill the remaining area with shorter paths.
- Never leave a dead-end cell. If a cell is surrounded on three sides by filled cells or walls, only one route can enter it. Check for dead-end cells and ensure a path visits them before they get cut off.
- Think in regions, not in lines. Divide the grid mentally into zones that each colour “owns.” A colour that has its dots in one corner should stay mostly in that corner; encroaching on another colour’s zone usually leads to a dead-end.
- Undo liberally. Flow Fill has unlimited undos. Use them aggressively rather than trying to fix a bad path by routing around it — one clean undo is faster than five corrective moves.
What this game trains your brain to do
Flow Fill belongs to the mathematical family of graph-embedding problems: given a set of nodes (the coloured dots), can you connect them with non-intersecting paths that cover every cell? In graph theory this is a form of constraint satisfaction problem (CSP), and many instances are NP-complete — meaning no polynomial-time algorithm is known for the general case. Humans solve these puzzles using spatial heuristics that differ dramatically from the brute-force search a computer would use, which makes Flow Fill an interesting window into human spatial cognition.
The cognitive demands of Flow Fill centre on prospective planning and working memory. To solve a puzzle you must mentally simulate several alternative paths simultaneously, compare them against the constraint of full coverage, and update your plan as each new path narrows the available space. This places a sustained load on the prefrontal cortex and the visuospatial sketchpad. Regular practice with grid-routing puzzles is associated with improvements in spatial reasoning scores and everyday tasks like navigation and technical drawing.
About Flow Fill
Flow Fill is one of the most universally accessible puzzle formats in the world — no reading required, no language barrier, playable by any age group with a touchscreen or mouse. The rules fit in one sentence, but the depth grows steadily across hundreds of levels without ever introducing new mechanics. Satisfying to casual players and genuinely challenging to puzzle enthusiasts.
All CrizBrain games including Flow Fill run entirely in your browser. No account, no install, no score data ever leaves your device. The puzzles are randomly generated each session so you always have a fresh challenge waiting, no matter how many times you have played before.