nose

[kb26]

Put your nose to the grindstone.

[pyoung]

Do you have a question about this?

[David L.]

Begin to work hard and continuously.

With exams in a month, a student may go to lectures, but decide not to go out at night, and spend each evening studying. He is concentrating on the study he needs to do. He has 'put his nose to the grindstone'.