Anyone who has written or done anything artistic seriously for any amount of time should know this. Ideas can come from anywhere at any time. Dreams, songs, bits of conversation, even seemingly from nowhere.
The problem is, how do we catch those ideas?
Someone, I think it was Brandon Sanderson, called it an ideas bucket. I have two general notebooks, one sits in the bag I always carry and the other is a small one that fits in my back pocket.
I have only recently started using the small one again and now I can’t live without it. Early on, when I first started carrying it, I used to think that I got no ideas while doing so. However, I started getting ideas from everywhere and, tired of writing on my hands all the time, I started carrying it again.
The bigger notebook is to ‘think out’ my ideas and write up some blog posts (like this one) at work. With more space, I can sit down and examine my ideas in more depth. I always use lined notebooks as I thin more in words than in pictures, though many times when thinking quickly and in a non-linear fashion, I write diagonally across the lines, and occasionally there is an image or two while thinking out scenes.
My point is, you have to have somewhere to keep your ideas. There is no way you can keep them all separated in your mind, so what do you use?