A teacher stands before his student, holding in one hand an empty jar and in the other a box of small rocks, each being 2 inches in size. I am going to pour the rocks into this jar and I would like you to tell me when it is full, he instructs the student.

Watching carefully, the student notices the rocks filling the jar. When it reaches the brim he nods in signal to the teacher, for he is proud of himself for noticing the apparent goal has been reached.

The teacher smiles, then puts down the box of rocks and instead picks up a box of pebbles. He gives the same instruction to the student, as he slowly starts pouring the pebbles into the jar of rocks. The pebbles seep through the air spaces between the rocks and after a short time, the student for a second time, signals that the jar is now full.

The teacher smies again, and then puts down the box of pebbles, replacing it with yet another box. This one contains sand. Again, he instructs the student to let him know when the jar is full, as he starts to pour the sand into the jar.

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