Age is just a number (really): It doesn’t actually reflect how old your body is, and, in fact, there’s no single answer to that question. As NPR’s Skunk Bear reports, each part of your body regenerates at a different rate. (We should note here, though—that seven-year thing is mostly a myth.)

In the video, host Adam Cole takes us on a brief journey around the body, explaining that while the cells in your stomach lining are constantly refreshed, the ones that make up your heart are much slower to change. From skin to hair to brain matter, everything is renewing at a different rate, and some cells are the very same ones you were born with.

“It’s just a matter of time before almost every part of your body refreshes itself,” Cole says.

To learn more about how quickly your body is renewing (and maybe give yourself an existential crisis in the process), check out the video here! And did you know that nuclear bomb testing informed our knowledge of cellular life spans?