Deep frying is a cooking method in which food is submerged in hot fat, most commonly oil, rather than the shallow oil used in conventional frying, done in a frying pan. Normally, a deep fryer or chip pan is used for this; industrially, a pressure fryer or vacuum fryer may be used. Deep frying may also be performed using oil that is heated in a pot. Deep frying is classified as hot-fat cooking method. Typically, deep frying foods cook quickly: all sides of a food are cooked simultaneously as oil has a high rate of heat conduction. The term deep frying and many modern deep-fried foods were not invented until the 19th century, but the practice has been around for millennia.