One day, a time management expert was lecturing to a group of business school students.
"We're going to do a little test," the expert said as he pulled out a one-liter wide-mouth jar and placed it on the table. Then, he took out a pile of fist-sized rocks and carefully placed them into the jar one by one until no more rocks would fit at the top. He asked, "Is the jar full?" All the students replied, "Yes, it's full." He countered, "Really?" Then, he reached under the table and brought out a bucket of gravel. He dumped some gravel in and shook the jar, allowing the gravel to fill the spaces between the rocks. "Now is it full?" This time, the students were more cautious. "Probably not yet," one student responded.