Five-year-old Thomas was terrified that at night, when his parents went to bed, a gigantic fox came out of hiding and sat next to the city, keeping watch.

In his mind the fox was as big as the city itself, and had no qualms about eating everyone and everything in it.

'Then why doesn't it?' asked Thomas's father. 'Eat the whole city, I mean.'

He doesn't want to, Thomas answered. He doesn't like the taste of people.

'So why are you scared?' asked his mother.

Because he still could, said Thomas.

'Where is the fox now?' they asked.

I don't know. I can't see him.

'You can't?'

No. He's invisible.

'So what makes you think he's there?'

Thomas could not answer this, but he also could not sleep. His parents soon learned that it is impossilbe to disprove the existence of a giant, invisible, harmless fox to a five-year-old.