Not long ago I googled why shit and piss are two things and not one. It just suddenly seemed odd to me one day. The answer seemed to be that the stuff we poo out never goes 'inside' our body. Imagining our bodies being like a tube or doughnut, what goes in our mouths just goes through the hole, staying in a sense on the outside. What comes out is just what we couldn't use as it passed along the conveyor belt. Piss, on the other hand, is the waste from (truly) inside us.

Hmm seems there are (at least) two levels of 'inside' here. It's quite a weird, topological subject hehe. It seems odd that we don't intuitively think of those as different degrees of insideness.

It's the end of the input. Think about it another way... say you chew on a slice of orange, swallow the juice, and spit out the pith. The pith isn't output of your body. It's just the end of the intake process.

Feaces isn't produced by your metabolic system. It's just the byproduct of intake. The stuff that your body didn't absorb.