Two little sheds sit out on the water, one black, one white, gabled like something from a child's drawing. They float. Licht Architecture anchored them to a pontoon on Lake Derby and ran a footbridge out from the shore, and the result was shortlisted at the national architecture awards. The sauna inside is lined in dark burnt larch. The changing room glows, its screening turning occupants into soft silhouettes. Lake Derby is not natural. It filled a tin mine that closed decades ago, and now it cools mountain bikers who plunge straight off the deck between rounds of heat. Architecture you can sweat in. Then jump.