To learn how to teach. Just because you know stuff, doesn't mean you know how to impart that knowledge to others. You also need how to develop lesson plans, make a syllabus for the school year ,and...
Assuming it's your son or relative, all you have to do is tell them how difficult it is in the real world and ask him where he pictures himself in 5 years. Then print out and hand him a Mcdonalds...