Epigenetics studies why genes turn on or off without any change to the underlying DNA sequence, and SciShow host Hank Green explains the mechanism alongside his animated clone, Circus Hank. The two walk through how chemical tags attached to DNA, added or removed by diet, stress, and other environmental exposures, can switch genes on or silence them, changing how a cell behaves without rewriting its genetic code. The episode uses this back-and-forth between the two Hanks to work through concrete examples of how identical twins can end up with different gene activity over a lifetime, and why the same genome can produce different outcomes depending on what a body encounters. It stays close to cellular biology rather than broader claims about identity or destiny, treating epigenetics as a layer of regulation sitting on top of inherited DNA. Runtime is short and the pace is quick, typical of the SciShow format.