Time moves forward and fixes your body into place. An embryo divides; one cell becomes many. These cells divide again; a brain, arms, and fingernails are formed. You’re born; you grow; you look and behave in ways that scream you. How does this happen? Your genes tell your body what to do, using proteins as instructions. But what tells your genes what to do? Among other things, your epigenes do. These are biochemical tags that switch your DNA’s protein-making ability on and off without changing the DNA itself.