A trainee school teacher was trying to get a class of small children to use words that began with specific letters, but the Headmistress had warned her that Little Johnny had a dirty mind and told her not to let him answer a single question.

"So, children", she began, "who can give me a word beginning with the letter A?"
Johnny has his hand up before she had finished asking the question.
"No! Not you, Johnny. Someone else may answer. "Yes Rachel?"
"Apple, Miss".
"Very good, Rachel!"
"Now, can anyone can give me a word beginning with B?"
Johnny has his hand up in a flash.
"No - sit down Johnny! Harry, you answer."
"Boat, Miss."
"Good. Now can anyone think of a word beginning with 'C?'. No! NOT YOU, JOHNNY! Jack, you try."
Jack said: "Cat!"
"Well done, now how about the letter 'F', anyone?"
Johnny was now so excited he could hardly contain himself but the teacher completely ignored him and addressed a pretty little girl sitting next to him. "You try, Amy."
"Farmer, Miss."
"Very good!"

The teacher continued all the way to the letter 'M', by which time Johnny was practically exploding with impatience. The teacher paused and glared at him, but since she couldn't think of any dirty words that began with the letter M, she finally called on him.
"So, Johnny, what begins with the letter 'M'?"
"Mirror, Miss!"
The teacher was visibly relieved. "Yes, Johnny, "Mirror, very good!"
Before she could get another word in, Johnny blurted out: "Amy has a mirror under her chair, Miss!"

The teacher was a bit taken aback, but decided to humour him. After all, there couldn't be anything remotely dirty about mirrors.
"Don't be silly, Johnny. Why do you think Amy has a mirror under her chair?"
"'Cos I can see myself in her knickers, Miss".