One day a time management expert was speaking to a group of business students, and to drive a point home, he used the following illustration:
As he stood in front of the group of high-powered overachievers he said, "Okay, time for a quiz."  Then he pulled out a one gallon, wide mouthed mason jar and set it on a table in front of him.  After which, he produced  about a dozen fist sized rocks and carefully placed them, one at a time, into the jar.
When he filled the jar to the top and no more rocks could fit inside, he asked, "Is this jar full?"
Everyone in the class said, "Yes."
Then he said, "Really?"
He reached under the table and pulled out a bucket of gravel. He then
dumped some gravel in and shook the jar causing pieces of gravel to work themselves down into the spaces between the big rocks.
He asked the group once more, "Is the jar full?"
By this time the class had awoken to his ploy.  "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 and it went into the spaces left between the rocks and the gravel.  Once more he asked the question, "Is this jar full?"
"No!" the class shouted.
Once again he said, "Good!"  Then he grabbed a pitcher of water and began to pour it in until the jar was filled to the brim, whereupon he looked up at the class and asked, "What is the point of this illustration?"
One eager beaver raised his hand and said, "The point is, no matter how full our schedule is, if you try really hard, you can always fit some more things into it!"
"No," the speaker replied, "that's not the point."
"The truth this illustration teaches us is:  If you don't put the big rocks in first, you'll never get them in at all.