The mysteries of poop answered

What is poop made of?

About 3/4 of your average turd is made of water. Of course, this value is highly variable - the water content of diarrhea is much higher, and the amount of water in poop that has been retained (voluntarily or otherwise) is lower. Water is absorbed out of fecal material as it passes through the intestine, so the longer a turd resides inside before emerging, the drier it will be.
Of the remaining portion of the turd, about 1/3 is composed of dead bacteria. These microcorpses come from the intestinal garden of microorganisms that assist us in the digestion of our food. Another 1/3 of the turd mass is made of stuff that we find indigestible, like cellulose, for instance. This indigestible material is called "fiber," and is useful in getting the turd to move along through the intestine, perhaps because it provides traction. The remaining portion of the turd is a mixture of fats such as cholesterol, inorganic salts like phosphates, live bacteria, dead cells and mucus from the lining of the intestine, and protein.