Production animals provide something that can be sold, other than their progeny. An example is sheep, which can be used to sell wool and meat. For accounting purposes, they are classified as non-current assets. The accumulated cost of production animals begins to be charged to expense through depreciation once the animals reach maturity. The depreciation period is the estimated productive lives of the animals. When a production animal is eventually sold, the accountant compares the price received to the net book value of the animal, and recognizes a gain or loss on the sale.