Now, The Rest Of The Story
Whenever I speak to adults, I always have to tell them the rest of the empty mayonnaise jar story:
After the professor was done with his life lesson, a student then stepped forward, took the jar, which the professor and other students agreed was full, and proceeded to pour in a glass of beer. Of course, the beer filled the remaining spaces making the jar truly full. (Why he had a bottle of beer with him in class I don't know; must have been a really boring professor.)
The morale of the story - no matter how full your life is, there's always room for beer!