As a comedian, I’m used to improvising, but my skills have never been tested as rigorously. It’s slightly easier, of course, to play a character when you’ve known them all your life, but still, you have to come up with explanations as to how your sledge flies or how you fit down the chimney. (For under-tens, ‘Magic dust’ answers everything; with older kids, a reference to Doctor Who and a temporal vortex does the trick.) At the end of the shift, though, when the beard was beginning to scratch and the 27th child in a row said no, he hadn’t been good, but he’d still like an iPad, my explanations got briefer.