A professor fills an empty mayonnaise jar with rocks to represent important people in a student's life, then adds pebbles to represent other priorities like work and home, and finally pours in sand to fill in the gaps to represent small stuff. He asks the students if the jar is full each time to show that space is limited. The professor explains that people need to focus first on the rocks, or most important relationships and aspects of life, before filling their time with less important sand, to have a full life with proper priorities.