Chances are good you think you're more or less the same person you were last week. But the lining of your gut is totally different, and the hairs on your head are 2.5 millimeters longer.
The human body's ability to replace worn out cells with shiny new ones is key to the long lifespans we're so used to. There are a couple things we keep all our lives, like the visual cortex, but almost everything wears out and gets replaced, at least for part of our lives. And some things, like our hair and nails, just grow and grow and grow.
We've gathered together scientists' estimates scientists of how quickly we go through different types of cells. Many of these ages have been established using a technique called bomb-pulse dating, which uses the traces of atomic radiation we each carry to determine how old cells are.
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