A grandpa and his grandson were out fishing and the grandpa pulls out a beer for himself. The boy asks if he could have some. His grandpa, knowing he shouldn't give it to the kid asks him a question in return. "Can you prick touch your asshole?" The boy says no so the grandpa says no.
This happens one more time with the grandpa's cigarettes. Eventually the grandpa is feeling tired and thirsty and sees that his grandson has some lemonade. Asking for it the grandson asks, "well, can your prick touch your asshole?" Proudly, the grandpa says, "why, yes it can." Hearing this, the grandson turns away and yells, "then go fuck yourself grandpa!"