There are many variations to this joke, but this is the latest (not the best).
A teacher asks the class if they had any relatives that fought in the war.
Little Johnny puts his hand up,
Johnny: "My grandad was in the war and got a grenade shoved up his ass by the enemy.
Teacher: "Rectum, Johnny, rectum."
Johnny: "Wrecked him? It nearly f'n killed him!!"