Johnny replies "We learned how to make babies!"
His mother, thinking she misunderstood asks him to repeat it.
"The teacher taught us how to make babies!"
His mother is naturally upset that the school should teach this to the children, without even asking her consent. She calls the school to complain, but they claim not to know what she is talking about. She turns back to Johnny.
"Are you sure they taught you how to make babies?"
"Yes" says Johnny. "You change the 'Y' to 'I' and add 'E' 'S'!"