I've never explained the "rules" I set out for myself in doing these simple sketches. The rules are simply a way to provide a simple structure within which to work, and have no particular significance:
I focus on something in my immediate surroundings, and keep it intimate: what I can take in at a glance. I allow myself to arrange things, as the main focus is always composition, both space and color, but not to fuss over it. I keep the time to under half an hour, and often find I spend less than that. But above all, I enjoy doing them. These are not a test, or an exercise so much as they are a way for me to bring my mind and my eye to the present moment. It is a peaceful way to end the day.