The world of science fiction is filled with countless other examples of mind-stretching technology. In the Harry Potter series, characters such as Albus Dumbledore store some of their memories externally in a device called the Pensieve. This chapter explores several potential technological applications that would enable us to extend and enhance our mental capacities: mind extensions, mind uploads, and mind mergers. The issue of personal identity over time – of what makes a person at one time identical to a person at some other time – is a complicated one. The chapter considers the case of conjoined twins Krista and Tatiana Hogan, who are fused at the skull. Craniopagus twins are extremely rare, occurring only once in every 2.5 million births; most such twins die within twenty-four hours of birth. Thinking about the possibility of mind merger raises questions about the unity of consciousness.