A tall tale
is a story that claims to explain the reason for some natural phenomenon, or sometimes illustrates how skilled/intelligent/powerful the subject of the tale was. In either case, the tall tale is fictional, and usually obviously so. Very often, the tall tale is told in a manner that is intentionally ridiculous.
The tall tale is a fundamental element of American folk literature. The tall tale's origins are seen in the bragging contests that often occurred when the rough men of the American frontier gathered. The tales of legendary figures of the American West -- such as Pecos Bill and Paul Bunyan -- owe much to the style of tall tales.