Guts. The tubes that transform delicious food into disgusting shit. Which, in turn, is delicious food to other creatures who turn it into even smaller shit. And so on until it is all recycled back into living tissue or fossilized. In nature as a whole there is no such thing as waste and if something can be digested and turned into energy to sustain life, sooner or later there will be an organism doing just that.
The interesting story about guts that our esteemed Professor Kos told was quite literally about shit.
Our digestive system is not particularly effective in absorbing fats, a significant portion of excrement are lipid compounds. And this, indirectly, is responsible for the oh so typical color of the final product of human digestive system.
When red blood cells die, the heme has to be broken down in order for the iron to be re-absorbed and recycled. Some of the end products of heme recycling are two chemicals: one called bilirubin (yellow), which gets later on broken down into stercobilin (brown). This is the reason why bruises go from initially red through blue to yellow and brown color as they heal.
The same process is happening also in liver and the chemicals bilirubin and stercobilin are excreted with bile. And because they are not water-soluble but are fat-soluble, they remain in the undigested fat in feces and are responsible for their distinctive color.