referring to the principle of suction).
One day a child came up to the teacher to inform her that one of the other students had said a bad word.
"What was the bad word he said?" asked the teacher.
"I can't say it."
"It's ok to tell me, you won't get in trouble for it."
"No, it's too bad, I don't want to say it."
"Well I have to know what he said in order to punish him. Can you tell me what it is without saying it?"
"Well... it rhymes with 'fuck'"!