About Glow Grid
Glow Grid is our version of an old toggle puzzle. Some tiles on a square board are lit and some are dark. Tapping a tile flips it along with the tiles touching its four sides (diagonals are left alone), and you win when every tile is lit.
There are 40 built-in levels: 6 on a 3x3 board, 8 on 4x4, 10 on 5x5, 8 on 6x6 and 8 on 7x7. Once you’ve worked through those, Endless mode deals random boards at any of the five sizes, and all five are open from the start.
Each board shows a par: the fewest taps that can solve it. Par isn’t an estimate. The game computes it exactly when the board loads, and we checked it against a brute-force search over every possible set of taps.
How to play
- Tap a tile. It flips, and so do its neighbors above, below, left and right. Edge tiles have three neighbors, corners two.
- Keep going until the whole board is lit. Every tap adds one to the move counter.
- Undo takes back your last tap and lowers the counter, all the way to the start if you want. Restart jumps straight back to the starting board.
- Clearing a level unlocks the next one.
Stars
| Result | Stars |
|---|---|
| Moves at par or fewer, no hint used | 3 |
| Moves up to par x 1.5 (rounded up) | 2 |
| Solved with more moves than that | 1 |
On a par-6 level, 6 moves gets three stars and 7 to 9 gets two. Since par is a true minimum, three stars means you matched it.
Hints
The hint button rings one tile that belongs to a shortest solution of the board as it is right now, and says how many presses are left, which can be more than the original par if you’ve wandered off course. Using a hint caps that run at two stars, and restarting or hitting Replay doesn’t lift the cap. Choosing the level again from the list does. With the keyboard cursor showing, H jumps the cursor to the ringed tile, so you can press H and then Space.
Two rules that make this puzzle make sense
Pressing a tile twice does nothing. A press flips the same five (or fewer) tiles every time. Flip a tile twice and it’s back where it started. So pressing tile X twice is the same as never pressing it, wherever those two presses happen in your sequence. Our undo button relies on this. It just presses your last tile again.
The order of presses doesn’t matter. A tile’s final state depends only on how many times it got flipped, and whether that count is odd or even. Pressing A then B flips the same tiles the same number of times as B then A. So any solution is really a set of tiles, each pressed once. You can press them in whatever order you like.
That leaves 2^25 possible press sets on a 25-tile board. Our solver treats each tile as an equation (“this tile needs an odd or even number of flips”) and solves all of them at once by Gaussian elimination, with 1 + 1 = 0 arithmetic. On 3x3, 6x6 and 7x7 there’s exactly one set of tiles that works, so par is just its size. On 4x4 there are 16 different sets that all solve a given board, and on 5x5 there are 4. The solver checks all of them and keeps the smallest. That’s why you can sometimes finish a 4x4 board in a completely different way from the hint and still hit par.
It also means most random 4x4 patterns can’t be solved at all: only 4,096 of the 65,536 possible patterns can, or 1 in 16. On 5x5 it’s 1 in 4. We never deal a dead board, because every level and Endless board is built by pressing tiles on a fully lit grid.
Strategy tips
Chase the dark tiles down the board
Go row by row from the top. For every dark tile in the top row, press the tile directly below it. That press lights the dark tile and doesn’t touch anything else in the top row. Then do the same for row two using row three, and keep going. When you reach the bottom, every row except the last will be lit.
On 4x4, chasing alone finishes the board
We tested this on every solvable 4x4 pattern. If the board can be solved, one pass of chasing solves it. It often costs more than par, though. On level 8, chasing takes 6 moves against a par of 4.
Learn the 5x5 top-row table
On 5x5, chasing usually leaves a few dark tiles in the bottom row. There are only seven patterns it can leave. Find yours below (X = dark), press the matching tiles in the top row, then chase again from the top.
| Bottom row left dark | Press in the top row |
|---|---|
| X…X | first and second |
| XX.XX | middle |
| .XX.X | first |
| X.XX. | last |
| ..XXX | fourth |
| XXX.. | second |
| .X.X. | first and fourth |
Clean up your solution afterward
Chasing gets you a finish, but rarely three stars. Once you’ve solved a level, think about which tiles you pressed an odd number of times. Those alone form a complete solution (on 4x4 and 5x5 not always the shortest one). Restart and press just those.
Look for plus shapes
Any dark plus shape (or a dark corner with its two neighbors) is one tap from lit. On 3x3 you can often see the whole answer as two or three overlapping pluses.
Endless mode
Endless boards press each tile of a lit board with a 50/50 chance, so they come out more scrambled than the levels. Easy ones get thrown out: the minimum par is 3 on 3x3, 4 on 4x4, 6 on 5x5, 8 on 6x6 and 10 on 7x7. In a test run of 1,000 boards per size, 7x7 pars ranged from 14 to 34 and averaged about 24. The last built-in level has a par of 20, for comparison.
FAQ
Why did I get two stars when I matched par?
You used a hint on that level at some point. Pick the level from the list to start a fresh run without the cap.
Does undo cost a move?
No. Undo removes your last tap from the counter, so a press plus its undo leaves no trace.
Is my progress saved?
Yes, in your browser: unlocked levels, best moves and stars, Endless counts, the sound setting and the board you were in the middle of. Clearing site data resets them.
