Imagine you are talking to a robot
You: Can you get me a glass of water?
Robot: Yes.
... you wait and wait and wait..finally realizing your mistake-
You: Will you get me a glass of water?
Robot: Yes.
... you wait and wait and wait
To ones who still haven't got it, the correct question should have been an order - "Get me a glass of water". A robot will never understand that he is supposed to get a glass of water even when literally you are just asking him if he can get water for you.

