Lights Out

Lights Out began life as a handheld electronic puzzle in the 1990s and became a favourite of mathematicians because such a simple rule hides real depth. The board is a grid of lights, some glowing and some dark. Pressing any light toggles it — and also flips its neighbours above, below, left and right. Every press changes up to five lights at once, which means the obvious move, pressing a lit light to switch it off, usually switches two others back on.

This version offers three grid sizes: 4×4 to learn the rules, the classic 5×5, and a 6×6 board that will test anyone. Every puzzle is generated by making random presses on an already-solved board, which guarantees a solution always exists — you will never be handed an impossible grid. Your only score is your move count, shown as you play, so a solved board invites an immediate rematch: can you do it in fewer presses?

Under the friendly surface sits genuine linear algebra — players and textbooks have analysed Lights Out for decades. The practical consequence is the famous light-chasing method described in the tips: solve the board row by row and let the last row tell you how the first row should have gone. Once that idea clicks, even the 6×6 board becomes a satisfying, methodical solve rather than a scramble.