If you chewed gum as a kid, chances are some well-meaning adult told you not to swallow it, lest it remain stuck in your digestive system for all eternity (or seven years, or some other lengthy period). That might be a pretty good way of convincing kids not to eat gum -- it worked on me as a child -- but if you're committed to the cause of truth, it's worth remembering that it is total BS.
Veteran myth-busting site Snopes recently updated its post on the oft-prevalent story, pointing out that it's not remotely backed up by biology:
This oft-repeated claim may stem from genuine confusion over a term commonly applied to chewing gum: indigestible. Although gum resists the body's efforts to break it down (hence the 'indigestible' designation), it does not linger in the stomach. Gum is eliminated as human waste in the same way, and at the same rate, as any other swallowed matter. Granted, it comes out the far end relatively unchanged by the trip, but it does come out on schedule.
That doesn't mean you'd want to encourage gum-swallowing, but it's not a particularly risky enterprise, or something to stress about as a one-off event.
The Seven-Year Glitch [Snopes]