At some point when I was young there was a bit of a craze for claiming that if you stood between two mirrors and managed to see 35 reflections of yourself, or whatever it was, then you would be sent off to fairyland. There were scary versions too, which held that if you stood looking into several mirrors at once as the clock struck 12 and recited the words “bloody Mary” three times then a demon would be summoned. In adulthood, this sounds more like an overreaction to a bad hangover. Nowadays, to enter a parallel universe, you don’t need...