Prince Harry urges people to “flex” their minds to unlock greatness. The Duke of Sussex, 37, reflected on the best way to achieve “peak mental fitness” as part of a new campaign to promote his $4.7 million (£3.8 million) Silicon Valley start-up. The dad-of-two – who has children Archie, three, and Lili, 13 months, with Meghan, Duchess of Sussex – said in a five-minute film: “We all have greatness within us. “Mental fitness helps us unlock it. It’s an ongoing practice, one where you approach your mind as something to flex, not fix.” He also talked about how he maintains his own mental fitness e…