Lights Out

Lights Out is a deceptively tidy little logic puzzle: a five-by-five grid of lights, some on and some off, and a single goal — switch every one of them off. The twist that makes it a puzzle rather than a chore is that each tap is contagious, flipping not just the light you touch but its four orthogonal neighbours too, so a careless press can relight half the board. Every layout here is scrambled from a solved state, so it can always be cleared; the sport is doing it in the fewest presses. The classic technique that tames it is "chasing the lights" — sweep top to bottom, and for every lit cell simply press the one directly below it, which marches all the remaining lights down into the bottom row where a known set of final presses finishes them off. It helps to remember that pressing any cell twice cancels out, so undoing your own moves is pure waste. Because a single tap changes up to five lights at once, planning a few presses ahead is what keeps you from flailing. Free, mobile-friendly, fewest-moves best saved.