One day Little Timmy woke up and, feeling particularly old that morning, ran down the stairs to his dad. Very excited, Little Timmy says "Dad! Dad! Do you know how old I am!?".
Dad was surprised but played along with his son and said "No, Timmy, how old are you?".
Very proudly Timmy exclaims "I'm nine!". "You're a big boy now Timmy aren't you!" replies dad.
With a feeling of success, Timmy enters the kitchen to meet his grandma. "Hey grandma! Do you know how old I am?". "No I don't, sweetheart" she says with a smile. "Really?" says Timmy.
At this point grandma has an idea. "I know how to find out, though". "How?" Timmy asks. "Unzip your pants and i'll show you".
So Timmy unzips his pants. Grandma then puts her hand into Little Timmy's Y-fronts and fondles his balls. After a minute or so she takes out her hand and says "You're nine aren't you".
Amazed, Timmy asks her how she knew. "I'll tell you how I knew, Timmy. I heard you tell your dad".