Mind One's P's and Q's

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Listen, Larry. If you want an invitation to Clarissa’s party you’d better mind your P’s and Q’s.

But I haven’t been doing anything to offend her.

I’ll tell you one thing. You’re going to have to be careful of what you say and how you act around Susan.

Come on! Susan and I are just friends.

I know that, but Clarissa is the jealous type. She’s liable to think that something is going on between the two of you.