I used to have a rule with my then 2 year old son about cussing. He was only allowed to cuss when he used it in a sentence, and not around his mother or teachers.
I went to pick him up from daycare and his teacher said when she went to pick him up earlier in the day he said, "Keep your fucking hands off me."
I laughed and said, "Good usage".
She was kind of a bitch anyway.
About 2 weeks later, he looked me straight in the eye and said "fuck you". I debated whether or not that was a sentence...the jury is still out, but regardless he lost his priviledge to cuss until he moves out of my house. He only has 9 years to go

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