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Kid Rock famously declared in American Bad Ass, "it stinks in here, 'cause I'm the shit." And, indeed, it is fairly universally noted that shit stinks (even if some people think theirs doesn't). But why does shit stink? Or, to put the inquiry in more scientific terms: why are our brains wired in such a way that we find ourselves repelled by the smell of the end product of processing the things we eat?

The answer lies, naturally, in chemistry and evolutionary biology.

Firstly, what is shit? There's all sorts of stuff in there. Whatever you've eaten, the non-liquid part that escaped every effort of your digestive tract to convert it into energy is in there, and if your body is working particularly inefficiently, it may be wasting up to half of what went in one end. But there's more to the story. A majority of fecal matter, by weight, is bacteria. Those wonderful critters that happily inhabit our intestines -- and provide invaluable aid in the breaking down and digesting of food -- are forever being flushed out and replenished. And it is they, more than anything we've eaten, that generate that smell, by releasing various sulfurous gasses.

The question then turns to why we perceive these gasses offensively, and the answer almost certainly lies in the fact that the same bacteria which help our digestion when kept to one part of the body will sicken, possibly kill us, if introduced to other areas. The greatest danger of wounds to the gut is septicemia. For humans, at least. Plenty of animals who eat shit all day long and seem to have no problem digesting it. But man has gone a different direction, a direction where we leave shit be, and consequently have no immunological acclimation against its hazards, substituting for this a much simpler mechanism to prevent the need for such an acclimation.

In other words, shit stinks (to us) because the happenstance of evolution has seen fit to make us into non-shit-eaters.

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