As a former professor, I want to second this advice. Even asking the professor to re-explain some from class is often helpful. If you run out of things to ask,

As a former professor, I want to second this advice. Even asking the professor to re-explain some from class is often helpful. If you run out of things to ask, ask about their research. Building that relationship will help if you need an extension on assignment, a bump in a grade, or a letter of recommendation later. Some professors won't care and it will be of little value. Do not get discouraged if it is not a positive experience. Find the professors that do care.