noun
a word that represents a person, place, thing, or idea
adjective
a word that describes or modifies a noun
article
a word that is used to specify a noun as definite or indefinite
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Her research methods were like a closed book to me.
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I don't understand calculus at all, it's like a closed book to me.
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Physics is a closed book for me, I just can't get it.
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His emotions are like a closed book, I can never tell what he's thinking.