I’m a new enough writer that I’ve only been asked that question a few times. Today, I’m going to answer it.
The shower.
Yes, when we bought our house, we didn’t know that the recently renovated bathroom was equipped with an Idea-matic 6000.
By the time I got out of the shower today, a novel idea I’ve had for awhile had spawned a more thorough history, a tense first chapter complete with dialog, and a beginning that might work well as a short story.
I’m not sure why showers do that, but other writers have mentioned doing good thinking in the shower. Maybe shampooing is just so boring that our brains have to come up with something, anything, to think about.
Now if only the shower also doubled as a time machine.
I used to do a lot of great thinking while mowing the yard. Same concept–you’re on autopilot so your brain is free to think of plots and characters and such.