When our DS was little, we did the usual emphasising the need to say the word 'please':
"Would you like a banana?"
"Yes!"
"Yes....PLEEEASE!!!"
He picked it up fairly quickly and we wanted to praise and positively encourage him, so we then had:
"Would you like a banana?"
"Yes, pleeease!"
"Good boy - nice manners!"
The problem was that he then believed that we were teaching him the second part of what he had to say to be polite, rather than what we said in response. Thus it became:
"Would you like a banana?"
"Yes, please, good boy, nice manners!"^
We chuckled and didn't correct him, but before long, we had the sad puppy-dog eyes looking up at us in bewilderment when we said No:
"Can I have a treat, yes please, good boy, nice manners?!"
"No, Sweetheart - you've not long had one. How about a banana or a carrot instead?"
"WAAAAH, BUT I SAID 'YES PLEASE, GOOD BOY, NICE MANNERS'!!!!"
Sorry, love, it's not a magic spell - it doesn't work quite as simply as that!