A philosophy professor stood before his class and had some items in front of him. When the class began, wordlessly he picked up a very large and empty mayonnaise jar and proceeded to fill it with rocks, about 2″in diameter. He then asked the students if the jar was full? They agreed that it was. So the professor then picked up a box of pebbles and poured them into the jar. He shook the jar lightly. The pebbles, of course, rolled into the open areas between the rocks. He then asked the students again if the jar was full. They agreed it was.
The professor picked up a box of sand and poured it into the jar. Of course, the sand filled up everything else. He then asked once more if the jar was full. The students responded with an unanimous - yes.