Did you know that we use the Genitive case in the meaning of “some” with uncountable nouns? When we use the Accusative case in the same situation, it has the same meaning as the definite article in English, the object is precise.
Я хочу воды. I want (to drink) some water.
Ты поела хлеба. You have eaten some bread.
Я хочу воду. I want (to drink) the water. (For example, from the glass I have).
Ты съела хлеб. You have eatent the bread. (For example, the slice of bread that was on the table).