Changelings often are faeries switched at birth with a human child. Some make deals with human children to trade places for a set time if the faerie is especially curious of the human world. There's almost no question that the human is curious of the fey world (much to their demise at times). Changelings, like all faeries, have a glamour to help disguise themselves as a mortal. Unless they are provoked and lose their concentration, no suspicion usually arises from mortals. Like all of fey, they are deathly allergic to iron, but can build up a tolerance to be near it if they spend enough time in the human world.