Anyone who has tried to build castles in dry sand knows the challenges desert digging spiders face. Because the grains don’t stick together, it’s hard to dig a burrow and keep its walls from collapsing. Even so, the "flic-flac" spider, Cebrennus rechenbergi—famous because it can cartwheel quickly across the desert—builds a tunnel 25 centimeters deep and uses it daily for shelter from Morocco's hot sun. Likewise, its neighbor, a newly discovered wolf spider called Evippomma rechenbergi makes tunnels, too. Yet they use different tactics, researchers report this week in the Journal of Arachnology. The flic-flac spider picks the right spot to dig, then pushes sand together and scoops it up in a basket formed by fine bristles that overlap. Lacking these bristles, the wolf spider instead glues sand particles together by connecting the grains with fine silk threads. Then it drags the silk-sand bundle away, the researchers report. Other spiders use combinations of these tactics, and one that lives in slightly moister areas will ball up the sand and hurl it like a shotput many centimeters from the burrow entrance. As for how to keep the tunnel walls from collapsing: The spiders studied incorporate silk threads into the tunnel as they build it, so it remains standing even if all the surrounding sand is removed. When it needs a new home, the flic-flac spider spends about 2 hours alternately digging and squeezing out the silk rings, then builds a lid. Those interconnected sand grains stabilize the walls, the researchers say. These discoveries help answer a long-standing mystery and could provide inspiration for engineers and castle builders, they note.