A philosophy professor stood before his class with some items on the table 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 inches 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.