Prototype

Parity

Press a cell to flip it and its four neighbours. Turn every light off. The twist: the order you press in never matters.

Parity map — what the board actually remembers

How to play
  • Every press flips the cell you pressed and its up/down/left/right neighbours.
  • The puzzle is built by pressing a hidden set of cells on a dark board, so pressing that same set again — in any order — solves it.
  • Pressing a cell twice is the same as never pressing it. The parity map below the board shows which cells you have pressed an odd number of times.
  • Hint points to a cell that is still needed. Undo is just another press of the same cell.

What it teaches: XOR, parity, and commutativity — the board is a sum of vectors over GF(2), so the solution is a set, not a sequence. Compare with Triad, where order is the entire puzzle.