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Towers of Silence: Zoroastrians consider a dead body - as also cut hair and nail-parings - to be nasu, unclean. According to tradition, the purpose of exposure is to preclude the pollution of earth or fire (see Zam and Atar respectively). Corpses are therefore placed atop a tower and so exposed to the sun and to birds of prey. Bodies are arranged in three rings: men around the outside, women in the second circle, and children in the innermost ring. The ritual precinct may only be entered by a special class of pallbearers. Once the bones have been bleached by the sun and wind, which can take as long as a year, they are collected in an ossuary pit at the center of the tower and/or are eventually washed out to sea.