Mindfulness of breathing - What is meant by "arising and passing away of the body"?

When it is said that one is "mindful of the arising and passing away of the body" -- what is meant with "being mindful of the arising and passing away of the body"? The body is here all the time, solid, and I am not understanding how it is "arising and passing away"? How can I see the impermanence of the body -- it is not the same as thoughts that can arise and pass, because the body is present here all the time.