Why when we doo-doo (move out bowls) why does it smell so badly? I understand that it's wast but what about it makes it smell so bad?

Certain classes of organic (carbon-containing) substances contain other elements that are particularly offensive to the HUMAN sense of smell. The elements roughly in the order ofdecreasing nastiness are: Sulfur, Nitrogen, and Phosphorous and combinations thereof. Probably the most familiar is hydrogen sulfide (the odor of rotten eggs) even though it does not contain any carbon. Others that you might be familiar with are butyl mercaptan C4H9--SH (skunk), and methyl mercaptan CH3--SH the chemical added to "natural" gas so that it is easily identified in case of a leaky gas main or a gas stove left on accidently. The various sulfur containing chemicals occur as part of the normal digestive processes in all mammals. The diet and the animal's particular digestive system determine the types and amounts of the very large number of possible odiferous substances.

If you can get past the smell there are several interesting questions that do not have entirely satisfactory answers. What is it in the evolution of humans that make certain substances so putrid smelling? Is there some evolutionary advantage to this sensitivity? I do not think anyone really knows (or should I say "nose") the answer.

Some substances (hydrogen sulfide is the classical example, but not the only one) are foul smelling at low concentration, but are less offensive, or even odorless at high concentration. In the case of hydrogen sulfide this is quite dangerous, because what you do not smell can definitely harm you. In addition, what humans find offensive other creatures find very attractive. In the case of feces flies are attracted quite strongly to that class of compounds. Other compounds (insect repellants are the classic example) are fairly odorless to humans, but very repellant to insects. Some animals (tracking dogs for example) can smell various "human" or other animal odors in incredibly small concentrations. In addition, there are cultural, or learned, differences in what some people find offensive, but others do not. These are but a few of the interesting aspects of odor, which is probably the least well understood of all the human senses.