"Put two fingers on the side of your neck. Feel that? That soft, insistent tap under your fingertip. That's happening right now, roughly once every second, and it's been happening without a single day off since before you were born. About three billion times over an average lifetime, if you're counting. I'm Priya, and that little drumbeat is where we start today. Mark is here. I do the counting, apparently. And I'm David, here to ask why a fist-sized muscle deserves a whole episode. Because that fist-sized..."