Many people are deplorably fascinated by the Australian Poo Machine ( Formally known as the Cloaca Professional, this contraption demonstrates what happens to food as it gurgles and peristalses through our digestive systems – from beginning to gooey, smelly end, in strict chronological ordure. I can't reveal which regular column artist begged to illustrate this. Nor which SFX editor mused, "Close encounters of the turd kind."

Of course this is relevant to SF. The genre's most sensitive treatment of such end-products must be The Dark Light Years by Brian Aldiss, offering the striking definition "Civilization is the distance man has placed between himself and his excreta." In the book, clean-limbed astronauts who believe this maxim are horrified by the alien utods who live far more natural, organic lives – closer to the nightsoil, you might say – and whose spaceships are veritable poo machines. Things end badly for the utods, who may be peaceful and unthreatening, but ... well, yuck.

Iain M. Banks's space opera Consider Phlebas opens with the hero about to drown in a rising, malodorous tide of sewage generated by revellers at a lavish banquet given in his honour. James Tiptree's "The Night-Blooming Saurian" features time travel to the dinosaur era, with a special emphasis on coprolites – not the usual fossils but messily fresh specimens. A Piers Anthony story regrettably titled "Up Schist Crick" climaxes with the dilemma of a chap who needs very urgently indeed to take a massive dump but is wearing a kind of skin-tight superhero costume and doesn't know how to take it off. Who said SF fails to address major contemporary issues?

Another such issue is where to locate the training ground for chaos wizards who must practice hurling devastating firebolts. L.E. Modesitt has a cunning answer in his fantasy The White Order: apprentice mages are assigned to sewer-cleaning duty, vaporizing revolting blockages while trying not to choke on the stench. Skilled operatives in Barry N. Malzberg's SF The Men Inside are miniaturized to perform approximately the same task within the natural sewage system of the human intestine, a job description best not contemplated. Returning to fantasy, kids obsessed with Number Twos – like Sam Vimes's precocious son in Terry Pratchett's Snuff – have long wondered about The House at Pooh Corner, and adults agree that the spinoff Cooking with Pooh is the worst cookbook title since James Beard's Beard on Pasta.

In Robertson Davies's semi-fantasy The Cornish Trilogy, there's a professor who spends his life studying the fascinating internal structure of human stools; even his eventual Nobel Prize hardly compensates for being called the Turd-Skinner. T.H. White's The Sword in the Stone introduces the wandering knight King Pellinore who's obsessed with his pursuit of the Questing Beast and always carries samples of this creature's fewmets or droppings to show his friends: "Yes, these are her fewmets." "Interestin' fewmets."

Thus reading T.H. White's Arthurian fantasy explains a warning letter from the Gandalf character in that notorious Tolkien parody, Bored of the Rings: "The halberd has fallen! The fewmets have hit the windmill!" Which reminds me that Roger Zelazny's Lord of Light – for the most part a fine serious SF novel – introduces a Hindu dignitary called the Shan of Irabek whose mind gets transferred into an epileptic body, all to set up the line: "Then the fit hit the Shan." Some readers never forgave the author.

A final classic of excretory SF is Damon Knight's "The Big Pat Boom", whose visiting aliens offer good money for ordinary cowpats. Instantly a school of cowflop connoisseurship springs up, with artistic Earthlings classifying the natural treasures as queen and even emperor pats (the emperor has a double whorl) and reverently hanging framed specimens over the mantelpiece. Why are those aliens laughing?

David Langford remembers that Pratchett's Vimes had such a deprived childhood that he had to play Poohsticks (invented by A.A. Milne) with real poo.