This is the 465th most frequent Lithuanian word.
vanduo
Water.
'Vanduo' is used as the subject of the sentence, referencing the concept of water in a general sense.
Vanduo yra būtinas gyvenimui.
Water is essential for life.
'Vandenį' is the accusative case of 'vanduo', indicating the object of the action (drank).
Aš gėriau šaltą vandenį.
I drank cold water.
'Vandeniu' is the instrumental case of 'vanduo', implying the means or accompaniment ('with water').