When I worked as a nanny, I once had a two-year-old point to her glass of water at lunch and demand, "what's that!?? When I explained that it was water, she looked at me like I'd just grown a second head, then she explained to me in a bland, slow tone, "I don't drink water. I only drink juice." (I blinked at her for a moment, then said, "well, today, you're drinking water." I don't remember if that worked or not.)
When my DD (16 mos) was born, I swore I'd never allow her to be a juice-addicted toddler -- heaven forbid someone has to have that conversation with my child!!! Every so often I'll have a tablespoon of juice or lemonade in my water glass, and DD usually gets a sip of that if she asks. But otherwise, it's only water in the sippy cup. I don't think she knows that other beverages are really an option.