Ropes are wound around a set of pins into a knot that looks impossible. Pull the pins out in the correct order and the whole thing falls apart cleanly. Pull them in the wrong order and it tightens.

Early levels have two or three ropes and give themselves away instantly. Later ones stack six or seven crossings deep, and the only way through is to trace a single rope from end to end and find the one pin that is not holding anything above it.

The rule that unlocks it: a pin can only come out if nothing crosses over it. So do not look for the pin that seems most tangled — look for the one that is clear on top. There is always exactly one, and removing it reveals the next.

No timer, unlimited attempts, and a proper sense of release when a knot you have stared at for two minutes finally drops.