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...