Often, they have unexpected twists. Just like in 'The Gift of the Magi', you think one thing is going to happen, but then it goes in a completely different, and humorous, direction. This keeps the reader on their toes.
They usually have relatable characters. In stories like 'Rip Van Winkle', Rip is the kind of character that many people can identify with - not perfect, a bit lazy perhaps. And his ordinary - man - in - extraordinary - circumstances situation creates a lot of comical moments. The use of American settings and dialects also adds to the humor. For example, Mark Twain's use of regional dialects in his stories makes the characters come alive and often leads to humorous exchanges.