Pigeons eating vomit...

... slowly freezing on the pavement. As Martin Amis said of pigeons, what is it that even they consider worth excreting? Another nasty thought is that most of what you excrete (anally) used to be part of yourself. I think we have a comforting fiction that it is undigested food or something. When it's undigested food you can see that it is (think sweetcorn) most of it is recycled cells and bacteria. If we produce a half litre of blood a week it has to go somewhere or we'd explode. Oh, and lots of bacteria too. Take some antibiotics and notice the effect. Mind you they do mess up your digestion anyway.