It's a praise that you receive from a teacher after you ask a question which he doesn't know how to answer, but he's too embarrassed to admit it.
Student: Teacher, how do you say 'lock, stock and barrel' in Spanish?
Teacher: Excellent question! It's a very interesting expression! For your homework, you'll have to look it up.
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an excellent question is a question that is very excellent
What is an excellent question is a excellent question.