How we check every puzzle

A good logic puzzle gives you very little and asks for one thing: the answer, reached without a guess. Here is how we make sure every puzzle on the site keeps that promise, and the one place it works differently.

Updated October 3, 2026

Exactly one answer

Every puzzle here is made by a program and then checked by a second one, a solver that counts the solutions. A puzzle with two answers is not a puzzle, so the generator throws it away and tries again.

• Queens, mini sudoku and nonograms: the solver must find exactly one solution before a puzzle is used.
• Knights and Knaves: every way of making each islander a knight or a knave is tried, and exactly one must fit what everyone says.
• The library (Star Battle, Nurikabe, LITS, Tapa and the rest): each puzzle is counted by an exact solver, and the stored sets of puzzles are checked entry by entry.

World is the exception. It’s a logic exercise, not a grid puzzle: you place pieces so a list of sentences is true, and often more than one board does that. Any board that makes every sentence true is correct, and the checker tells you which sentence fails if one does.

No guessing

One answer isn’t enough. You should be able to reach it by reasoning. So for the daily set (Queens, mini sudoku, the nonogram and Knights and Knaves) the generator also runs a second solver that only uses the moves a person would: a region that fits in one row, the only digit left, the overlap of a long run, a case followed to its end. If that solver can’t finish, the puzzle is thrown away, however unique it is.

The hardest steps are sometimes a supposition: suppose this square were filled, follow what it forces, and it breaks a rule, so it isn’t filled. That is proof by contradiction, the same move a mathematician makes. It is logic, not a guess. You never have to try something and hope. Our lesson Deduce, don’t guess explains the difference.

Checked a year ahead

Before each release, a set of checks re-solves a full year of daily puzzles in every game, and the release stops if a single one has a second solution, or, in the daily set, can’t be finished by logic. The same checks run the hints across weeks of puzzles to make sure each one is a sound move.

Hints that reason

When you ask for a hint, it first checks your marks. If one is wrong, it tells you which, because no deduction can start from a mistake. Otherwise it names the next move it can prove from the board in front of you, shows where to look, and only places it if you ask.

Lessons computed, not typed

In Learn, every truth value in a drill (is this sentence true on this board?) is worked out by our logic engine when the site is built, never typed by hand. Every Knights and Knaves example is solved by trying all the cases. A wrong answer option has to be wrong on some board, or the build fails.

Who makes it

Logic Puzzle Club is made by James Murray. If you find a puzzle with two answers, or a hint that doesn’t follow, tell us from the puzzle’s page with “Report a problem”. We read every report.