Stephen Covey in his book, First Things First, shares the following story experienced by one of his associates:
I attended a seminar once where the instructor was lecturing on time. At one point, he said, "OK, time for a quiz." He reached under the table and pulled out a wide-mouthed gallon jar. He set it on the table next to a platter with some fist-sized rocks on it. "How many of these rocks do you think we can get in the jar?" he asked.
After we made our guess, he said, "OK. Let's find out." H e set one rock in the jar . . . then another . . . then another. I don't remember how many he got in, but he got the jar full. Then he asked, "Is this jar full?" Everyone looked at the rocks and said, "Yes."
Then he said, "Ahhh" He reached under the table and pulled out a bucket of gravel. Then he dumped some gravel in and shook the jar and the gravel went in all the little spaces left by the big rocks. Then he grinned and said once more, "Is the jar full?"
By this time the class was on to him. "Probably not," we said. "Good!" he replied. He reached under the table and brought out a bucket of sand. He started dumping the sand in and it went into all of the little spaces left by the rocks and the gravel. Once more he looked and said, "Is this jar full?" "No!" we roared.
He said, "Good!" and he grabbed a pitcher of water and began to pour it in. He got something like a quart of water in that jar. Then he said, " Well, what's the point?" Somebody said, "Well, there are gaps, and if you work really hard you can always fit some more things into your life."
"No," he said, "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?"
Source: First things First - Stephen Covey