A river city with two islands and seven bridges, after Königsberg. Every district has an odd number of bridges, so no walk can cross each one exactly once. The walker tries anyway: six bridges in, it is stuck on the north bank, feels its way back over every bridge it has already used, and the walk rewinds. Then an eighth bridge builds itself, girder by girder. Two districts turn even, the dial drops from four odd to two, and the same walker crosses all eight in one go. Around the map, probes fire rays and count the shores they cross: odd means inside an island, even means outside. One bridge is all it takes for parity to change its mind.