Can You Guess

Q: can you guess what i am. you always hold me but you cant see me. unly way you can see me is if im looking at me. you use me everyday withought knowing. i hurt with bright lights whcih makes me get bigger. i make water out of thin air but you rub it then its no longer there. when you use me you give me a "stare" but when you dont use me. you dont know im there.

A: your eye balls.