Sometimes the computer eats them. Sometimes they're scribbled in a notebook or on a piece of scratch paper and then forgotten. Sometimes they never exist outside my brain because when I go to write them down or type them out, they've vanished. One time I had a computer disk stolen that had the only copy of two chapters of a novel. The rest of it was carefully backed up, but not those two chapters.
Several years ago I attempted to transcribe all the little bits and pieces of half-started novels and put them in one file on the computer: NEW BOOK IDEAS. Seriously, folks, you don't want to know how many folders are in that file. Gazillions. And there are a bunch of others that I never had time to transcribe.
One of my (many) problems is that I can come up with half a dozen titles at the drop of a hat, and then I want to write books to go with the titles. Some of the folders in that digital file contain no more than a page or two of vague ideas, but I liked the title.
A couple of days ago I went looking for the original hard copy version of one of my published novels and discovered that it has disappeared. Even though I have two other hard copies as well as two other versions on disk, that original original original is . . . . gone. So I have four versions plus the actual printed edition (which is slightly different even from those) but the original is lost, probably forever. I wanted one scene from it to resurrect into a revised digital edition and now I'll have to live without it.
It doesn't pay to get too attached to our words.