Afraid of the dark
Our son suddenly became afraid of the dark one day. It was like it hadn't occurred to him before then. I'm not sure what triggered it. We told him that it was okay to be scared, that lots of people were scared of the dark, but that there wasn't really anything to be afraid of. We told him that monsters were only pretend - that they were only on TV and in books. He accepted all this, but usually had to remind himself, "There's no monsters," if we were in the car after dark, or if the wind was particularly noisy at night.
The other night he asked to go outside. This is not unusual, as he pretty much wants to go outside at all times. But it was after dark.
I didn't really think about it at first, but after wandering with him for a little while outside, I asked, "You're not afraid of the dark anymore, huh?"
"No," he said. "There's no monsters."
"Yeah, monsters are just pretend, right?"
"Yeah, I threw them in the trash."