A boy sees his father smoking a cigarette.
"Dad, can I have a puff of your cigarette?"
"Well, son, can your dick reach your asshole?"
"No."
"Then you can't have a puff of my cigarette."

The boy sees his father drinking a beer.
"Dad, can I have a sip of your beer?"
"Well, son, can your dick reach your asshole?"
"No."
"Then you can't have a sip of my beer."

The boy sees his father reading a porn magazine.
"Dad, can I take a look at your magazine?"
"Well, son, can your dick reach your asshole?"
"No."
"Then you can't take a look at my magazine."

The boy, angry and frustrated, walks over to the kitchen to grab the last piece of a delicious chocolate cake. His father walks in.
"Son, that cake looks mighty good.. think I could have a bite?"
"Well, dad, can your dick reach your asshole?"
His father smiles and goes, "Why yes, as a matter of fact it can."
"Well then dad, you can go fuck yourself."