Authentic bird's-nest soup is made from nests of some species of swiftlet, mainly the edible-nest (or white-nest) swiftlet ("Aerodramus fuciphagus") and the black-nest swiftlet. Instead of twigs, feathers and straw, these swiftlets make their nest only from strands of their gummy saliva, which hardens when exposed to air. Once the nests are harvested, they are cleaned and sold to restaurants. Eating swiftlet nest material is believed to help maintain skin tone, balance "qi" ("life energy") and reinforce the immune system. It is also believed to strengthen the lungs and prevent coughs, improve the constitution and prolong life. The nutritional value of 100 g of dry nest includes 49.9 g of water-soluble protein (including amido nitrogen, monoamine nitrogen, non-amino nitrogen, arginine, humin, histidine, lysine and cysteine), 30.6 g carbohydrate (glycoprotein and mucin), 4.9 g iron, 2.5 g inorganic salt (including potassium, sodium, calcium, magnesium, sulfur, phosphorus, silica and other trace elements), and 1.4 g fiber (Dictionary of Traditional Chinese Medicine, The History of Chinese Medicine and the Nutrition Table).