I have always thought of this as my first insight in a GTD approach to work; It’s worth going back and reading “Time Management: The Pickle Jar Theory” from 2002,
“Each of us has many large priorities in our life, represented by the large rocks. We also have things which we enjoy doing, such as the pebbles. We have other things we have to do, like the sand. And finally, we have things that simply clutter up our lives and get in everywhere: water.” — Time Management: The Pickle Jar Theory