The question as to the essential nature of fairies has been the topic of myths, stories, and scholarly papers for a very long time.
Practical beliefs and protection
When considered as beings that a person might actually encounter, fairies were noted for their mischief and malice. Some pranks ascribed to them, such as tangling the hair of sleepers into "Elf-locks", stealing small items or leading a traveler astray, are generally harmless. But far more dangerous behaviors were also attributed to fairies. Any form of sudden death might stem from a fairy kidnapping, with the apparent corpse being a wooden stand-in with the appearance of the kidnapped person. Consumption (tuberculosis) was sometimes blamed on the fairies forcing young men and women to dance at revels every night, causing them to waste away from lack of rest. Fairies riding domestic animals, such as cows or pigs or ducks, could cause paralysis or mysterious illnesses.As a consequence, practical considerations of fairies have normally been advice on averting them. In terms of protective charms, cold iron is the most familiar, but other things are regarded as detrimental to the fairies: wearing clothing inside out, running water, bells (especially church bells), St. John's wort, and four-leaf clovers, among others. Some lore is contradictory, such as rowan trees in some tales being sacred to the fairies, and in other tales being protection against them. In Newfoundland folklore, the most popular type of fairy protection is bread, varying from stale bread to hard tack or a slice of fresh home-made bread. The belief that bread has some sort of special power is an ancient one. Bread is associated with the home and the hearth, as well as with industry and the taming of nature, and as such, seems to be disliked by some types of fairies. On the other hand, in much of the Celtic folklore, baked goods are a traditional offering to the folk, as are cream and butter. The prototype of food, and therefore a symbol of life, bread was one of the commonest protections against fairies. Before going out into a fairy-haunted place, it was customary to put a piece of dry bread in one’s pocket.” Bells also have an ambiguous role; while they protect against fairies, the fairies riding on horseback — such as the fairy queen — often have bells on their harness. This may be a distinguishing trait between the Seelie Court from the Unseelie Court, such that fairies use them to protect themselves from more wicked members of their race. Another ambiguous piece of folklore revolves about poultry: a cock's crow drove away fairies, but other tales recount fairies keeping poultry. In County Wexford, Ireland, in 1882, it was reported that “if an infant is carried out after dark a piece of bread is wrapped in its bib or dress, and this protects it from any witchcraft or evil.”
While many fairies will confuse travelers on the path, the will o' the wisp can be avoided by not following it. Certain locations, known to be haunts of fairies, are to be avoided; C. S. Lewis reported hearing of a cottage more feared for its reported fairies than its reported ghost. In particular, digging in fairy hills was unwise. Paths that the fairies travel are also wise to avoid. Home-owners have knocked corners from houses because the corner blocked the fairy path,and cottages have been built with the front and back doors in line, so that the owners could, in need, leave them both open and let the fairies troop through all night.Locations such as fairy forts were left undisturbed; even cutting brush on fairy forts was reputed to be the death of those who performed the act. Fairy trees, such as thorn trees, were dangerous to chop down; one such tree was left alone in Scotland, though it prevented a road being widened for seventy years. Good house-keeping could keep brownies from spiteful actions, because if they did not think the house is clean enough, they pinched people in their sleep. Such water hags as Peg Powler and Jenny Greenteeth, prone to drowning people, could be avoided by avoiding the bodies of water they inhabit. Other actions were believed to offend fairies. Brownies were known to be driven off by being given clothing, though some folktales recounted that they were offended by inferior quality of the garments given, and others merely stated it, some even recounting that the brownie was delighted with the gift and left with it. Other brownies left households or farms because they heard a complaint, or a compliment. People who saw the fairies were advised not to look closely, because they resented infringements on their privacy. The need to not offend them could lead to problems: one farmer found that fairies threshed his corn, but the threshing continued after all his corn was gone, and he concluded that they were stealing from his neighbors, leaving him the choice between offending them, dangerous in itself, and profiting by the theft. Millers were thought by the Scots to be "no canny", owing to their ability to control the forces of nature, such as fire in the kiln, water in the burn, and for being able to set machinery a-whirring. Superstitious communities sometimes believed that the miller must be in league with the fairies. In Scotland fairies were often mischievous and to be feared. No one dared to set foot in the mill or kiln at night as it was known that the fairies brought their corn to be milled after dark. So long as the locals believed this then the miller could sleep secure in the knowledge that his stores were not being robbed. John Fraser, the miller of Whitehill claimed to have hidden and watched the fairies trying unsuccessfully to work the mill. He said he decided to come out of hiding and help them, upon which one of the fairy women gave him a gowpen (double handful of meal) and told him to put it in his empty girnal (store), saying that the store would remain full for a long time, no matter how much he took out. It is also believed that to know the name of a particular fairy could summon it to you and force it to do your bidding. The name could be used as an insult towards the fairy in question, but it could also rather contradictorily be used to grant powers and gifts to the user.