The NYT recently featured an article, Your Body Is Younger Than You Think, which explores the speedy turnover rate of our cells:

Although people may think of their body as a fairly permanent structure, most of it is in a state of constant flux as old cells are discarded and new ones generated in their place. Each kind of tissue has its own turnover time, depending in part on the workload endured by its cells. The cells lining the stomach, as mentioned, last only five days. The red blood cells, bruised and battered after traveling nearly 1,000 miles through the maze of the body's circulatory system, last only 120 days or so on average before being dispatched to their graveyard in the spleen.

This reinforces the sentence I find myself using often: The body is a verb, not a noun.

Our bodies are like rivers, constantly changing. But there are some things that remain the same: even though our cells keep sloughing off and regenerating, the new cells hold the memories of the old cells. Our entire history is recorded inside those little luminous globs. Someone can touch our back a certain way, and a memory comes flooding through us, even though those original back cells are long gone. This isn't really covered in the article, but it's something I've explored myself and find so utterly fascinating.

I found this sentence of the article quite poetic:

About the only pieces of the body that last a lifetime, on present evidence, seem to be the neurons of the cerebral cortex, the inner lens cells of the eye and perhaps the muscle cells of the heart.

The sites of thinking and seeing and feeling--it makes sense that those are the places that stay intact our entire lives. Maybe it's those unchanging things that give us a coherent sense of self, of soul, even. But I think it's probably our shifting cells that are most in tune with the rest of the continually changing world.