Corner flood grid: rules and printable materials

Change the color of the corner-connected region to absorb its neighbors and cover a grid. Symbols keep it usable without color printing.

Players

1–6 cooperative

Plan for

10–15 minutes

Bring

A 4 by 4 symbol grid and two pencil marks

Use symbols as colors

The starter grid has four rows: AABB, ACBB, CCDD, CDDD, read left to right from the top. A, B, C and D are color names represented by letters, so colored ink is unnecessary. Begin with the connected A region containing the top-left square. Connection means shared sides, never just a corner. Keep a separate working copy and the printed original.

Recolor the whole region

Choose a different letter and change every square currently connected to the top-left square into that letter. Any side-touching region already bearing that letter is absorbed, including any further squares connected through it. A move affects the entire existing region at once, not a single chosen square. You cannot recolor an isolated distant island or count a diagonal touch as a bridge.

Check the exact example

From the starter, choose C: the A region absorbs the connected C squares. Then choose D to absorb the D region. Finally choose B to cover all sixteen squares. The route C,D,B uses three moves. Since the grid contains four letters and the start is A, at least three changes are needed to absorb all other letters; this route therefore also proves the starter minimum.

Play with a recorder

One person selects the next letter, a second traces all absorbed squares, and a recorder writes the move count and region size. Agree the boundary before changing marks. There is no speed requirement. If a region size appears to shrink, restore the previous row and check for a missed connection: the corner territory can only stay the same size or grow.

Create a fresh board

Shuffle letters on a new four-by-four board after the demonstration. Keep the initial board and complete move log if comparing solutions, since another board may need more moves. Do not claim the three-move key solves arbitrary grids. Use three letters for a lighter version and a larger grid for a longer shared session. A helper can trace connections while the participant chooses letters.

Printable materials

Preview at 100% scale, with browser headers and footers off. Cut with suitable help. Keep host keys separate. This is a manual tabletop kit, not an online opponent, automatic judge or solver.