[Bargain corner] Amazing Animal Engineers

Animals build an incredible array of structures in which to sleep, raise young, or catch prey. Some structures involve excavating in subsoil or wood, others involve construction underwater, and some are fixed to trees. Polar bears dig out caves in the snow to protect themselves from the coldest weather. Spiders spin webs of incredibly strong silk to ensnare prey. Prairie dogs dig underground 'towns' that can house hundreds of families.