Boots are usually defined simply as shoes whose uppers reach above the ankle. Boots are eminently practical and were usually developed for specialized functions: they protect the lower extremities against all manner of injury, from brambles and snakebites to weaponry; a well-oiled boot is waterproof and useful to mariners, fishermen, and dredgers; and a well-constructed riding boot can protect a rider's leg from the weight of a fallen horse. Most boot designs developed from existing shoe designs. Arctic boots, however, seem to have developed from sandals. As human beings entered the extreme North they extended and curled the bottoms of their sandals to protect their feet and added a sock or leggings. As time went on, they began to sew the sock directly to the edges of the sandal. The remnants of the sandal are often seen in the straps still affixed to the edge of the sole.