A fly refers to the outer layer of a tent or to a piece of material which is strung up using rope as a minimalist, stand-alone shelter. In basic terms, a fly is a tent without walls. Purpose-made stand-alone flies are also sometimes referred to as bivouacs, bivvies, tarpaulins, or hootchies. Flies are generally used for keeping moisture (such as condensation or rain) or sun off people while they eat, rest or sleep. They can also be used as groundsheets, but this is not recommended since it creates wear and tear which can lead to holes.