I heard this from a friend at Duke University:

Six year-old boy to his father: Daddy, can I have a train set?
His Father: Does your dick touch your ass?
Boy, reaching into his pants: No.
His Father: Well, come back when it does.

Five years pass.
Eleven year-old boy to his father: Dad, can I have a ten speed bike?
His Father: Does your dick touch your ass?
Boy, reaching into his pants: No.
His Father: Well, come back when it does.

Another five years passes.
Sixteen year-old boy to his father: Dad, can I have a car?
His Father: Does your dick touch your ass?
Boy: Yes, it does.
Father: Good. Then go fuck yourself.