When I was 11 two things happened to me: I climbed a tall tree in a storm, and I watched the film Alien. One of these things gave me nightmares for years afterwards; the other was scary, exhilarating and life-affirming. Which one was unsafe? As you may have guessed, it was my time on the claustrophobic Nostromo with Sigourney Weaver and John Hurt’s little friend that traumatised me. I can still recall a particularly gruelling Alien nightmare that I had in my early 20s. I’ve watched the film subsequently, as well as a couple of the sequels, but at 11 I was unprepared for the psychological shock I received. Clinging to the top branches of a poplar tree as it tossed in a fierce gale, I knew I was fairly safe. I’d done this kind of thing before; I knew my limits; and climbing that tree was me pushing past them gently.