When my daughter was little, and troubled by night-time fears, and I still had my magic powers, I used to enclose her in a magic bubble. She had to lie very still while I slowly and carefully drew the outline of the bubble, taking great care when it came to sealing the two ends together. Like most bubbles it only had a limited life, and had evaporated by morning, but during the night she was invulnerable. I had forgotten all about it, until I learned that, nearly thirty years on, she helps her little nephew to sleep the same way.