There is not even a proper, polite term for it. You can talk about it in clinical or scatological terms, but whichever expression you use it sounds wrong, either too coarse or too coy.
By now, I don't even have to say what this natural function is. You already know.
But one doctor, the gastroenterologist and nutrition expert Dr Anthony Leeds, of King's College London, has grasped the nettle, and has been studying stools for the past 20 years.
He believes the state of the stool can tell us a great deal about internal and digestive health. As a result, he and colleagues have developed the Bristol Stool Chart, named after the city where it was devised. The chart is based on clay models of different types, ranging from 'smooth soft sausages' to 'fluffy pieces with ragged edges'.
In the days before scientific medicine, doctors would ask patients about their bowel function as a matter of course. Examining these offerings continues to be an integral part of much traditional medicine, notably Chinese. But with the advent of tests and hi-tech instruments, examining the stool has fallen out of favour. Dr Leeds hopes that his work, and that of his colleagues in the Department of Nutrition and Dietetics, will restore the practice.
'We're a long way from having a completely scientific approach,' he said. 'It's difficult to overcome embarrassment and reluctance in this area, and I keep quiet about what I do at dinner parties. But the condition and weight of the stool can tell us much about a patient's general health.
'Stool weights below 100g (3.5oz) a day are associated with chronic constipation, and weights below 150g a day increase the risk of colon cancer. We also know that nearly half of the population of the UK passes stool weights below 50g per day. Although colon cancer is relatively rare, chronic constipation is associated with diverticular disease, which causes acute abdominal pain. Haemorrhoids, plus varicose veins in other areas, such as the leg, can also result from chronic constipation.'
But what is chronic constipation? There are several definitions. One is opening the bowels less than daily, but most specialists believe it is the kind of stool, and difficulty in passing it, that constitutes constipation.
'On the chart, passing stools of types 1 and 2 - hard pellets or hard, lumpy sausages, would count as constipation. Very few people will go to their doctors with constipation unless it gets very bad indeed, but it can lead to serious disease and it is also extremely easy to treat.'
Laxatives are still popular, but Dr Leeds believes that the best way to overcome constipation is to increase intake of fibre. 'Although we've been hearing about fibre for more than a decade now, as a nation we've hardly increased our intake at all. We still need to eat more fruit and vegetables and increase our intake of high-fibre breakfast cereals.
'Increasing fibre intake from 13g to 18g a day - which isn't much - increases average stool weight by 25 per cent. There is an important relationship between fibre intake and prevention of colon cancer. We are still a long way from optimum fibre targets in this country. 'We know that energy intake is going down, as people do ever less active jobs, but we are still not eating enough fibre.
'Most people become constipated on very low-calorie diets but, as Audrey Eyton said in her famous F-Plan diet, the best way to lose weight is to eat more fibre. Dietary fibre helps excess calories to be eliminated.'
But the weight of the stool is not the only important consideration. Transit times have to be taken into account. Dr Leeds is working on a 'foolproof' method of establishing transit times, using sweetcorn as a marker. 'The optimum transit time is less than 50 hours, with 20 to 50 as average. Transit times of less than 20 hours are too fast and indicate that food is not being properly absorbed, whereas anything higher than 50 hours indicates constipation.'
Dr Leeds's studies have established that lack of fibre is not the only cause of constipation: there are psychological aspects too. Humans like to establish regular habits for opening their bowels. As animals like to defecate in the same corner of their cage, or the same part of the garden or field, so humans feel most comfortable using the same lavatory, at the same time, every day.
Confronted with strange apparatus, or put out of routine, the bowels seize up. 'Almost everybody agrees that you can't go when removed from your daily routine,' Dr Leeds said. 'Most of us can't go in friends' houses, whereas nobody minds urinating. It may come down to the fact that the sites animals use are marked as part of their territory, and humans do the same. This means we can't easily go on somebody else's territory.'
There are dramatic differences between men and women here. 'On the whole, men find it easier to go, and they also do bigger stools than women. This is partly because they eat more, and also eat more fibre, and partly because they seem less hung up about it,' Dr Leeds explained.
'Our latest figures, based on questionnaires, reveal that 6 per cent of men and 19 per cent of women go less than daily, and 14 per cent of men compared with 26 per cent of women pass hard, strained stools. Women complain more about constipation, and whereas only 2 per cent of men would consider themselves constipated, 10 per cent of women believe they have a chronic problem.'
Dr Leeds admits that he does not really know why we are so embarrassed about this essential function. After all, we can live without sex, but we cannot live without moving our bowels several times a week. 'People are always grateful if we can solve their bowel problems, and that's why we are developing the stool chart and the transit-time test for GPs to use. It's often said that half of the world looks to see what they've done, and the other half never does.
'Animals always look, and so should humans. If we could overcome our inhibitions, we could prevent many digestive diseases.'
Finally, what is the ideal stool? According to Dr Leeds, that one is easy to answer: it looks like a sausage or snake and is smooth and soft and easy to pass. It is known as type four on the stool chart, and if you're passing these regularly, your bowels - and probably the rest of you - are in glowing health.