"How?" you're asking.
It is because stomach acid is strong enough to dissolve metal. The digestive juices in your stomach include an extremely corrosive acid called hydrochloric acid. If given enough time, this chemical has the ability to dissolve many different types of metals.
Frenchman Michel Lotito was known for publicly ingesting objects that you would think couldn't be digested. This strange ability stemmed from the fact that his stomach lining was twice as thick as that of an average human being. He had the ability to digest up to 2 lbs of metal daily!
Among his claims to fame are that he has eaten television sets, shopping carts, a coffin, 18 bicycles, and of course, an airplane. Strangely enough he couldn't eat bananas or hard-boiled eggs.
(Sources: 1, 2)