A professor stood before his class, picked up a large jar and filled it with golf balls. He asked his students if the jar was full. They agreed it was.
Then he poured pebbles into the jar. The pebbles rolled into the open areas between the golf balls. Again he asked if the jar was full, and the students responded, “Yes.” Next he poured sand into the jar, and it filled up everything else. Again he asked the students if the jar was full, and they said, “Yes.”