For most people, the only time they’ve seen the inside of an air duct is in a movie. The hero, locked in a room, must climb through the opening of an air duct that’s conveniently affixed to the wall or ceiling.
Sorry to spoil your fun, but in reality, air ducts are really small. They’re even smaller in homes, usually made of flexible tubing with a typical diameter of only 12 inches. Worst of all, duct leaks don’t make noise or produce any obvious symptoms. It won’t be until you receive your energy bill and notice the increase that you’ll suspect a leak could be involved.
(Plus, trying to force yourself through a duct could disconnect it, and that would definitely cause a duct leak!)