In September 1811 William Burchell, collecting in South Africa, picked up what he took for a curiously shaped pebble and found it was a plant. A lithops is a pair of fat leaves sunk in the ground with little more than the flat tops showing, and those tops are translucent, so the story tells itself: they are skylights, letting the sun down to a green interior hiding underground. In 2000 two researchers put reflective tape over the windows of three desert succulents, one of them a lithops. The plants with open windows did no significantly better. Later papers are still working out what the windows are for. It got past Burchell by looking like a stone, and it may have got past the rest of us by looking like a house.