Play Hidden balls laser deduction puzzle (Black Box) online Works — plays right here, unlimited puzzles
Guide updated October 2026 · Runs via WebAssembly (native port)
Black Box is a deduction game. Several balls are hidden in a square arena, and the only way to find them is to fire laser beams in from the edges and see where they come out. A ball straight ahead absorbs a beam, a ball diagonally ahead turns it. Mark where you think the balls are, then ask for a check. The puzzle is ready below; click an edge square to fire.
Controls — what does what
| Action | Keyboard | Mouse / touch |
|---|---|---|
| Fire a laser | Arrows to an edge square, Enter | Left-click an edge square |
| Place or remove a ball guess | Enter inside the arena | Left-click inside the arena |
| Lock a square, row or column | Space | Right-click |
| Check your guesses | Move to the check button, Enter | Click the button at top left once enough balls are placed |
| Undo / redo | U or Ctrl+Z / R or Ctrl+R | Game menu |
FREE This is the complete game, free. Open source under the MIT licence: the complete puzzle, with a new grid every time.
Black Box is from Simon Tatham's Portable Puzzle Collection, © Simon Tatham and contributors, used under the MIT licence. Built from source by scripts/build-puzzles.mjs.
How to play
- A beam travels straight until something affects it.
- A ball directly in its path absorbs it: shown as H (hit).
- A ball diagonally ahead on one side turns the beam 90 degrees away from that ball.
- A beam that comes back out where it went in, or is turned before it even enters, is shown as R (reflected).
- Any other beam leaves somewhere else; its entry and exit are marked with the same number.
- Place your guessed balls; you win if your layout gives the same results as the real one for every possible beam.
Sizes and difficulty
The Type menu offers 5x5 with 3 balls, 8x8 with 5 balls, 8x8 with 3 to 6 balls, 10x10 with 5 balls and 10x10 with 4 to 10 balls. When a range is given you do not know exactly how many balls there are. Custom sets the size and the number or range of balls.
Solving tips
An R usually means a ball sits diagonally in front of the entry point, in the first row or column of the arena. An H means a ball is somewhere along that straight line before any turn. Beams that pass straight through a row or column clear every square on it. Each beam you fire is information, so use them to settle the uncertain spots before checking.
Where this puzzle comes from
Black Box is part of Simon Tatham's Portable Puzzle Collection, a set of about forty logic puzzles written by Simon Tatham and contributors and released under the MIT licence. Every grid is generated on the spot, so the supply never runs out, and for the grid-logic puzzles the generator makes sure each has exactly one solution, reachable by reasoning rather than guessing. The version here is the collection's own code compiled to WebAssembly; the Game menu can save a puzzle to a file and load it again, and the Type menu changes the size and difficulty.
Frequently asked questions
What happens if my guess is wrong?
The game shows the least it needs to prove you wrong: either it circles a beam you fired that your layout contradicts, or it reveals one new beam, in red, that your layout gets wrong. You can then try again.
Can there be more than one right answer?
Yes, with five or more balls some layouts give identical results for every beam. The game accepts any layout that matches all beams, not only the one it chose.
Is Black Box free to play here?
Yes, with no limit on the number of puzzles. Black Box is open-source software under the MIT licence, and every grid is generated fresh when you press New game.
Does it work on a phone or tablet?
Yes. A tap works as a click and the puzzle resizes to fit the screen. Puzzles whose controls use the right mouse button are easier with a mouse.
More you can play in your browser
Related guides
- MinesweeperThe Minesweeper you remember — three board sizes, chording, first click always safe. Insta
- Code-breaking colour game (Guess)Play Guess free online: work out the hidden row of coloured pegs from black and white…
- Haunted mirror maze puzzle (Undead)Play Undead free online: place ghosts, vampires and zombies in a maze of mirrors so every
- Mosaic puzzle (Mosaic)Play Mosaic free in your browser: shade squares so every clue matches the number of black
Want a game like this on your own site?
Hidden balls laser deduction puzzle (Black Box) isn't ours to give away — we host it, we didn't write it. But eleven of the games here we did write from scratch, and those are free for anyone to put on their own page: one line of HTML, no account, no permission, no attribution beyond a credit line.