Published on Piff the Puffin, August 22, 2007.
One day a wise teacher was speaking to a group of his students. He pulled out a one-gallon, wide-mouthed mason jar and set it on a table in front of him. Then he produced about a dozen fist-sized rocks and carefully placed them, one at a time, into the jar.
When the jar was filled to the top and no more rocks would fit inside, he asked, “Is this jar full?”
Everyone in the class said, “Yes.”
“Really?” he asked. “Let’s see.” He reached under the table and pulled out a bucket of gravel. Then he dumped some gravel in and shook the jar, causing pieces of gravel to work themselves down into the spaces between the big rocks. Looking carefully from face to face, he smiled benevolently and asked again, “Is the jar full?”
His class was catching on quickly. “Probably not,” one of them answered … (full text).