Poultry feed is food for farm poultry, including chickens, broiler chicks, geese and other domestic birds. Feed for poultry mostly consists of grain. A portion of commercial feed, typically around a quarter, is known as bulk and is indigestible. The amount of bulk is referred to as bulk density. The quantity of feed, and the nutritional requirements of the feed, depend on the weight and the age of the poultry as well as the season. Healthy poultry require a sufficient amount of protein and carbohydrates, along with the necessary vitamins, dietary minerals, and an adequate supply of water. Certain diets also require the use of grit, tiny rocks such as pieces of granite, in the feed. Grit aids in digestion by grinding food as it passes through the gizzard. Grit is not needed if commercial feed is used.