Evil Grandma Joke

One day, a boy wakes his father up and says: "Daddy! Daddy! Today's my birthday! Guess how old I am!"
"I Don't know son, how old are you?"
"I'm eleven years old!"
"That's amazing. Now go tell your mother in the living room, or something."
So the boy find's his mother and says: "Mommy! Mommy! Guess how old I am!"
"I don't know."
"I'm eleven years old!"
"Great, now go tell your grandmother in the kitchen."
So the boy finds his grandmother and says:"Grandma! Today is my birthday! Guess how old I am!"
So Grandma unzips the boy's pants and with her cold, wrinkley, gnarled hand begins feeling the boy's scrotum. After some time the grandmother takes her hand out and says: "You are eleven years old."
"How could you possibly know that?!" replies the dumbfounded and violated boy.
"I heard you tell your mother..."