Prism

Prism is a light-routing puzzle on a grid. A source fires a beam; you place mirrors, beam splitters, colour filters and dispersing prisms to steer that beam into every receiver on the board. The board re-solves the instant you touch anything, and the new light sweeps out from the source so you can watch cause become effect.

The catch is colour. Receivers want an exact colour and nothing else — a red receiver rejects white. Filters subtract, prisms take white apart into red, green and blue, and two beams landing on the same receiver add together, so red plus green is yellow and all three is white again. Most of the difficulty is not aiming, it is arithmetic: working out where a colour has to be made, and where it has to be thrown away.

There are 26 hand-built levels. Each new piece arrives in a level that cannot be solved without understanding it, and the levels after it complicate that understanding. Early boards take seconds. Later ones ask you to put one colour in two places at once, or to notice that a filter belongs before a splitter rather than after it.

There is no timer and no move limit. You can undo, reset, and wander back through the level grid whenever you like. The only score is the number of pieces you used, kept per level, if you want a reason to go back and do it more cleanly.

Every solution here has been solved by hand and checked by an exhaustive search, so nothing on the board is unfair or accidentally impossible.