Some of earth's creatures create structures using unbelievable materials. Prairie dogs build extensive air-conditioned, underground cities with mounded entrances. The burying beetle drags animal carcasses underground for an edible nursery. Adélie penguins build stone beds to raise their eggs above the cold ground. A decorator crab camouflages its body with seaweed. Parrot fish build mucous sleeping bags and cave swiftlets use sticky saliva to build nests in caves.