Beginning of the terrible two's??

While we were on our Christmas vacation at my parent's house, we had our first taste of the terrible two's from our little girl (who is 21 months old). She was doing something she knew she shouldn't and every time I would correct her she'd scream at me.

Every time, we talked about how that's not how you talk to Mommy, but it didn't seem to make an impression.

Since we've been home, it has happened once or twice, usually when it's time to do something she'd rather not. Are there any suggestions for how to discipline this behavior?