Many children hear, "If you swallow gum, it stays in your stomach for seven years!" This is an urban legend. It is true that your body cannot digest the base of gum.

But we eat many things that we cannot digest, like corn skins and seeds. The muscles of the intestine push them along. A piece of gum usually leaves the body in a few days. But swallowing a lot of gum every day can cause problems.

"Swallowed gum stays in your stomach for seven years" is a well-known warning, but it is an urban legend. Part of it is true: gum base, the part that does not dissolve when you chew, cannot be digested. However, being indigestible does not mean staying forever. We regularly eat things we cannot digest, such as corn skins and seeds, and they pass through us. The intestines move food along with wave-like muscle movements, and when the stomach is empty, strong cleaning waves push leftover material forward.

So a single piece of gum usually leaves the body within a few days. However, a 1998 report described two children who developed blockages because they swallowed gum every day. The seven-year story probably began as a way to stop children from swallowing gum.

Generations of children have been told that swallowed gum sits in the stomach for seven years. Nobody has ever found a scientific source for that number, though the warning does start from something real. Gum base, the resin-like material that makes gum chewy, resists our digestive enzymes and is essentially indigestible.

But food that cannot be digested does not have to stay put. Corn kernels, tough vegetable fibers and fruit seeds also travel through us undigested, and they do not pile up. Peristalsis, the wave-like contraction of the gut wall, keeps everything moving. Between meals, a strong pattern of contractions called the migrating motor complex sweeps leftover solids out of the stomach and further down the tract. A single piece of gum normally leaves the body within a few days.

Trouble comes from habit. A 1998 paper in Pediatrics described children who developed bezoars, compact masses that can block the digestive tract, after swallowing large amounts of gum day after day, sometimes together with objects such as coins. These were individual case reports, so they cannot tell us how common such blockages are. And in two of the three cases, the child had been swallowing gum again and again.