Overheard as Ariana (5), Joel (3) and Elena (9 months) were playing:
Ariana: Elena, please don't grab my blocks. Could you ask permission? Say, "May I please use that block?" It is much more polite that way.
Ariana (more firmly): Elena, you just took my pink flower block. Please listen to me. You need to ask for permission before you take something. Just ask me and Joel if you want something of ours. I will share with you. (pause) But I don't think Joel would. Joel, would you share with Elena if she asks politely?"
Joel (matter-of-factly): No, I won't. It has corn for me. (Joel cites his corn allergy whenever he finds anything unpleasant from which he would prefer to be exempted).
Ariana (sorrowfully): Yes, that is what I thought. But I will share. Now, Baby, say "please".
Me: Sweetheart, Elena doesn't know how to say that yet. She is still little and is just learning to talk.
Ariana (patiently): OK, then, Elena say "puh--llll-eeee-z" (enunciating very slowly and clearly). And you should also apologize for knocking over Joel's tower. He feels angry when you knock it down.
I found this pretty funny, especially since we have never made a big deal about the kids saying please. We do try to model it, and both of our verbal children use "please" and "thank you". I didn't expect them to be actively teaching it to each other, but Ariana is quite comfortable in the role of big sister.