Do you know that story about the wolf and the seven kids, recorded by the brothers Jacob and Wilhelm Grimm? (If you don’t, you can read it here.) At the end [*spoiler alert*] the mother goat fills the wolf’s stomach with stones and stitches it up.

Fairytales aren’t the only place that stones are found in stomachs. And real stomach-stones were once thought to have magical properties. There are two types, each with their own name: bezoars and gastroliths.

Bezoars are hard lumps that form in the intestines of animals (often horses) and even humans. Their medical name is enterolith. They aren’t stones from the outside world but build up inside the gut from minerals in food.

In the Middle Ages, people believed that bezoars could protect them against any type of poison. They sold for lots of money and were looked after carefully, though getting them out of the half-digested mush in a dead animal’s stomach can’t have been a nice job. Careers you really don’t want: bezoar-hunter.

The  French surgeon Ambroise Paré wasn’t so sure about bezoars and their magical abilities.  So in 1575 he put them to the test. A cook had been sentenced to death by hanging, and Paré persuaded him to act as guinea pig. It must have seemed a good deal to the chef. If the bezoar worked, he would survive and be set free; if it didn’t – well, he was going to die anyway, so what did it matter? The cook was fed poison (mercury chloride) from a glass containing a bezoar and everyone waited to see what would happen. It wasn’t good – the cook soon found out why it mattered: he died in agony seven hours later, begging to be hanged after all. But the king wasn’t convinced. He was  cross that his particular bezoar was obviously a fake, but continued to believe that ‘real’ bezoars would work against poison.

The phrase ‘caveat emptor’ (buyer beware) was first used in English law in 1603 in a case in which a seller of fake bezoars was prosecuted.

In parts of China, people still encourage the formation of bezoars in unfortunate yaks. The bezoars are removed and sold as medicine. People can develop bezoars from eating too many unripe persimons, popcorn or chewing gum. Or their own hair. So don’t.

The other type of stomach stones is gastroliths. These start off as stones and are swallowed by the animal, either on purpose or when scooping up food. It’s thought that gastroliths help to break up food in the animal’s gut. The churning action of the gut tumbles the stones around with the food, helping to smash it up.  Some water-going animals seem to swallow stones to help them control their buoyancy – whether they float or sink.  Modern crocodiles, extinct plesiosaurs and perhaps some tadpoles do (or did) this. Sauropods – giant plant-eating dinosaurs like Diplodocus – possibly also ate stones, but it’s not clear whether they did it on purpose or just scooped them up accidentally with their food. Some stones were up to 10 cm (4 inches) long. The total mass of stones found  in the stomach region of one dinosaur fossil (a Cedarosaurus) was 7 kg! We can’t be sure it ate them, but it would have been a heavy load if it did.