Hermit crabs sometimes move house all at once. When one finds an empty shell that is too big for it, it may wait beside it, and others gather, and they line up roughly by size, smallest at one end, sometimes holding on to one another. Then a crab big enough for the empty shell arrives and climbs in, and the whole queue goes in a few seconds: each crab steps into the shell the one ahead of it has just left. One empty house, and everyone in the line moves up. The only shell left over at the end is the smallest, the one nobody needed. It must be the only property market where waiting in line actually works.