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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.