Solving serious puzzles — to catch criminals and thwart terrorist plots — is what the men and women who work at GCHQ do round the clock. It’s hardly surprising that many of them enjoy setting and solving them in their own time, too, pitting their wits against each other. This selection is from The GCHQ Puzzle Book (Penguin), put together by GCHQ’s spies, containing a Christmas puzzle challenge, too, and raising money for the Duke and Duchess of Cambridge’s Heads Together mental health campaign.