I Just Can't

While waiting for our train ride home, and thirsty after a long trip around the city, Mommy asked Pia if she had any more water in her water bottle, which she carries in her bag.

Mommy: Pia, do you have water in your water bottle?
Pia: Yes
Mommy: Can you give me some?
Pia: I cannot.
Mommy: Why not?
Pia: Because it's mine.