One day, a teacher entered her classroom and placed a glass jar on the table. She put two large rocks into the jar until no more could fit.
She asked the class if the jar was full and they all said, “Yes.” She responded, “Really?” She pulled out a pile of small pebbles, added them to the jar, and shook it slightly until they filled the spaces between the rocks.
Then she asked again, “Is the jar full?” The students agreed that it was. Next, she added a scoop of sand to the jar. She filled the space between the pebbles and asked the question again.
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