People that do not write have a hard time understanding that ideas are everywhere. They’ll come to you and say “I’ve got this great idea for a novel, I’ll tell you so you’ll write it and we can share the money.”
They get it wrong on three counts – first, because they think there really is any serious money in writing (ah!), and second, they believe their idea is unique (it’s not).
Third, and final, you can’t write a book based on a single idea. You need at least two good ideas to rub together for a long work to have a hope in hell.

Ideas are everywhere, and a good writer – well, a decent writer… let’s say a serviceable hack – is the one that can recognize them as they pour around him.
A general rule of thumb is, when you are overworked, stretched thin and at the lowest point of a low period, you’ll start getting all these brilliant ideas.
It’s like an Egyptian curse.

Let me give you an example.