Young Johnny is at school and he has to give a talk on what he did over the weekend.

He gets up before the class and proceeds to tell the entire class how he and his best mate, Freddy, spent the weekend catching frogs, shoving crackers up their arses and lighting them.

At this point the Teacher, a sweet, demure young lady, says to Johnny, "Rectum, Johnny, Rectum."

At which Johnny turns to her and says, "Sure did Miss.  Blew the little buggers to bits!"