Stephen Covey tells the story in First Things First of attending a seminar in which the instructor pulled out a wide-mouth gallon jar. The instructor placed the jar on the table next to some fist-sized rocks.
“How many of these rocks do you think we can get in the jar?” he asked.
The students made various guesses. The instructor then proceeded to fill the jar with the rocks. When the jar appeared to be full, he asked the class, “Is this jar full?” Everyone looked at the jar and agreed that it was indeed full.
The instructor then reached under the table and pulled out a bucket of gravel. He then dumped the gravel into the jar. The gravel went in between all the little places left by the big rocks.
The instructor grinned and once more asked, “Is the jar full?” By this time, the class was on to him. “Probably not,” several of the students said.
“Good!” he replied. The instructor reached under the table and brought out a bucket of sand. He poured it into the jar. It went into all the spaces left by the big rocks and the gravel. Again, he asked the class, “Is this jar full?”
“No,” the class shouted.
The instructor said, “Good!” He then grabbed a pitcher of water and poured almost a quart of water into the jar. Then he said, “What’s the point?”
Someone said, “If you really work at it, you can always squeeze more stuff into your life.” “No,” the instructor responded. “That’s not the point. The point is this: if you hadn’t put these big rocks in first, would you ever have gotten any of them in?”
I would make an additional point. The big rocks are a metaphor for the important stuff. If you don’t make room for the important stuff, it will be overwhelmed by the less important stuff.