An overnight guest remarks on a number of small, brightly coloured toothbrushes resting in a small beaker beneath the sink.

"They're for my children." I tell her. "They are ghosts. I mean, they may be ghosts but still, clean teeth are important, especially for children."

It doesn't occur to my guest to ask their names.

"I'm only joking," I tell her, "they're not ghosts, they're invisible."