What's mine is yours

Poor Cassia. Have you ever tried to teach a toddler the concept of mine and yours, me and you? I must say the thought had occurred to me as I watched my nieces grow up that it must be a difficult one, but they appear to have caught onto the idea OK. But Cassia's having trouble at the moment. Anything I sit on, hold, touch or otherwise engage with is "mine". As in, she says "mine chair." And yesterday we had an unfortunate nappy-changing accident in which I scratched her on the leg and she went around saying "cetchoo, cetchoo" ("I scratched you") while pointing to her injury. How exactly am I supposed to deal with this one? I think it's probably best to just leave it and she'll get the hang of it by osmosis eventually.