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The word faeces is the plural of the Latin word fæx meaning "dregs". There is no singular form in the English language, making it a plurale tantum. There are many colloquial terms for feces, of which some are considered profane (such as shit) while others (such as poo, poop, number two, deuce, doodoo, dookie, and doody) are not. Terms such as dung, scat, spoor and droppings are normally used to refer to non-human animal feces.
Stool is a common term normally used in reference to human feces. For example, in medicine to diagnose the presence or absence of a medical condition, a stool sample is sometimes requested for testing purposes. The term "stool" can also be used for that of non-human species.
After an animal has digested eaten material, the remains of that material are expelled from its body as waste. Though it is lower in energy than the food it came from, feces may still contain a large amount of energy, often 50% of that of the original food. This means that of all food eaten, a significant amount of energy remains for the decomposers of ecosystems. Many organisms feed on feces, from bacteria to fungi to insects such as dung beetles, which can sense odors from long distances. Some may specialize in feces, while others may eat other foods as well. Feces serve not only as a basic food, but also a supplement to the usual diet of some animals. This is known as coprophagia, and occurs in various animal species such as young elephants eating their mother's feces to gain essential gut flora, or by other animals such as dogs, rabbits, and monkeys.
Seeds may also be found in feces. Animals that eat fruit are known as frugivores. The advantage for a plant in having fruit is that animals will eat the fruit and unknowingly disperse the seed in doing so. This mode of seed dispersal is highly successful, as seeds dispersed around the base of a plant are unlikely to succeed and are often subject to heavy predation. Provided the seed can withstand the pathway through the digestive system, it is not only likely to be far away from the parent plant, but is even provided with its own fertilizer.
Organisms which subsist on dead organic matter or detritus are known as detritivores, and play an important role in ecosystems by recycling organic matter back into a simpler form which plants and other autotrophs may once again absorb. This cycling of matter is known as the biogeochemical cycle. To maintain nutrients in soil it is therefore important that feces return to the area from which they came, which is not always the case in human society where food may be transported from rural areas to urban populations and then feces disposed of into a river or sea.
In humans, defecation may occur (depending on the individual and the circumstances) from once every two or three days to several times a day. Extensive hardening of the feces may cause prolonged interruption in the routine and is called constipation.
Human fecal matter varies significantly in appearance, depending on diet and health. Normally it is semisolid, with a mucus coating. Its brown coloration comes from a combination of bile and bilirubin, which comes from dead red blood cells.
In newborn babies, fecal matter is initially yellow/green after the meconium. This coloration comes from the presence of bile alone. In time, as the body starts expelling bilirubin from dead red blood cells, it acquires its familiar brown appearance, unless the baby is breast feeding, in which case it remains soft, pale yellowish, and not completely malodorous until the baby begins to eat significant amounts of other food.
Throughout the life of an ordinary human, one may experience many types of feces. A "green" stool is from rapid transit of feces through the intestines (or the consumption of certain blue or green food dyes in quantity), and "clay-like" appearance to the feces is the result of a lack of bilirubin.
Bile overload is very rare, and not a health threat. Problems as simple as serious diarrhea can cause blood in one's stool. Black stools caused by blood usually indicate a problem in the intestines (the black is digested blood), whereas red streaks of blood in stool are usually caused by bleeding in the rectum or anus.
Food may sometimes make an appearance in the feces. Common undigested foods found in human feces are seeds, nuts, corn and beans, mainly because of their high dietary fiber content. Beets may turn feces different hues of red. Artificial food coloring in some processed foods such as highly colorful packaged breakfast cereals can also cause unusual feces coloring if eaten in sufficient quantities.
Laboratory examination of feces, usually termed as stool examination or stool test, is done for the sake of diagnosis, for example, to detect presence of parasites such as pinworms and/or their eggs (ova) or to detect disease spreading bacteria.
The distinctive odor of feces is due to bacterial action. Gut flora produce compounds such as indole, skatole, and thiols (sulfur-containing compounds), as well as the inorganic gas hydrogen sulfide. These are the same compounds that are responsible for the odor of flatulence. Consumption of foods with spices may result in the spices being undigested and adding to the odor of feces. The perceived bad odor of feces has been hypothesized to be a deterrent for humans, as consuming or touching it may result in sickness or infection. Human perception of the odor is a subjective matter; an animal that eats feces may be attracted to their odor.
Pets can be trained to use litter boxes or wait to be allowed outside and defecate there. Training can be done in several ways, especially dependent on species. An example is crate training for dogs. Several companies market carpet cleaning products aimed at pet owners.
Manure and fertilizer
Human feces may be used as fertilizer in the form of biosolids (treated sewage sludge). The feces of animals are often used as fertilizer; see manure and guano. In northern Thailand, elephants are used to digest coffee beans for Black Ivory coffee; the worlds most expensive blend of coffee.
Animals such as the giant panda  and zebra  possess gut bacteria which are capable of producing biofuel. The bacteria Brocadia anammoxidans, can create the rocket fuel hydrazine from feces. 
Gut flora transplant
Some animal feces, especially those of camel, bison and cattle, are used as fuel when dried out. Animal dung, besides being used as fuel, is occasionally used as a cement to make adobe mudbrick huts or even in throwing sports such as cow pat throwing or camel dung throwing contests.
Kopi Luwak (pronounced [ˈkopi ˈlu.aʔ]), or Civet coffee, is coffee made from coffee berries which have been eaten by and passed through the digestive tract of the Asian Palm Civet (Paradoxurus hermaphroditus). Giant pandas provide fertilizer for the worlds most expensive green tea. 
Dog feces were used in the tanning process of leather during the Victorian era. Collected dog feces were mixed with water to form a substance known as "bate". Enzymes in the dog feces helped to relax the fibrous structure of the hide before the final stages of tanning.
Feces have always been associated with the lowest people among society, the social outcasts, the pariahs, and the social discards. The Caste system in India was created along the lines of profession and the dalits (untouchables) were left to do work related to human emissions. They did such work as cleaning and picking feces from streets, cleaning toilets, and working with dead bodies. Such practices are prevalent even today in the rural and small villages of India.
The feces of animals often have special names. For example:
- Non-human animals generally –
- As bulk material – dung
- Individually – droppings
- Cattle –
- Bulk material – cow dung
- Individual droppings – cow pats, meadow muffins etc.
- Deer (and formerly other quarry animals) – fewmets.
- Wild carnivores – scat.
- Otter – spraint.
- Birds (individual) – droppings (also include urine as white crystals of uric acid).
- Seabirds or bats (large accumulations) – guano.
- Herbivorous insects, such as caterpillars and leaf beetles – frass.
- Earthworms, lugworms etc. – worm castings (feces extruded at ground surface).
- Feces when used as fertilizer (usually mixed with animal bedding and urine) – manure.
- Horses – horse manure, roadapple.
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