A harmonica has free reeds that vibrate in an airstream and they do not (unless there is a problem with the harp) come into contact with other components.

• 汽车引擎有很多的零部件，它们需要一段时间的磨合才能很好地协同工作，但口琴只有一个在气流中振动的自由簧片，除非故障，簧片不会与任何其他部件接触（也就不存在磨合的问题）。