I’m sure by now it’s an urban myth, but the story goes that there was a teacher who had a container. He filled it with rocks and asked the class if the container was full. They responded “yes.” He then took sand and poured it into the container. The sand filled the space between the rocks. Again he asked his class if the container was full. Again the class affirmed that statement. Finally he poured in water. The water filled the space between the sand.