Why is it that I get great writing ideas when I can't write?

I distinctly remember telling myself: that's a great idea! remember that idea when you get out of the car and write yourself a note so that you can put that in your blog.

But do I remember the IDEA that I was telling myself to write about? NOOOO!

Why do BRILLIANT ideas occur to me when I am driving? Or otherwise engaged so that I cannot write down the germ of the idea?

Just a phrase or two would capture it; but try as I might to chant the phrase internally, it still dissolves like morning mist before I find pen and paper.