The math professor’s six-year-old son knocks at the door of his father’s study.
“Daddy”, he says. “I need help with a math problem I couldn’t do at school.”
“Sure”, the father says and smiles. “Just tell me what’s bothering you.”
“Well, it’s a really hard problem: There are four ducks swimming in a pond, when two more ducks come and join them. How many ducks are now swimming in the pond?”
The professor stares at his son with disbelief: “You couldn’t do that?! All you need to know is that 4 + 2 = 6!”
“Do you think, I’m stupid?! Of course, I know that 4 + 2 = 6. But what does this have to do with ducks!?”