One day, an expert in time management was speaking to a group of students and, to drive home a point, used an illustration those students will never forget.
As he stood in front of the group of overachievers he said, “OK, time for a quiz.” He pulled out a one-gallon, wide-mouth jar and set it on the table in front of him. He also produced about a dozen fist-sized rocks and carefully placed them, one at a time, into the jar. When the jar was filled to the top and no more rocks would fit inside, he asked, “Is this jar full?”
Everyone in the class yelled, “Yes.” The time management expert replied, “Really?” He reached under the table and pulled out a bucket of gravel. He dumped some gravel in and shook the jar, causing pieces of gravel to work themselves down into the spaces between the big rocks. He then asked the group once more, “Is this jar full?”
By this time the class was on to him. “Probably not,” one of them answered. “Good!” he replied. He reached under the table and brought out a bucket of sand. He started dumping the sand in the jar and it went into all of the spaces left between the rocks and the gravel. Once more he asked the question, “Is this jar full?”