While you were in the womb, cells from your mom took up residence inside you. And cells from your baby body transferred to your mom as well, settling down in her tissues and organs. In this episode we talk with evolutionary biologist Amy Boddy about this phenomenon - called maternal-fetal microchimerism - and what it means for our understanding of who we are. If you've had a baby (or a few babies), cells from those babies may be inside your body and brain right now, proliferating and interacting with the rest of you. What are they doing and why? Listen and find out!