open one's eyes
open one's eyes or open up one's eyes {v. phr.}
To make a person see or understand the truth; make a person realize; tell a person what is really happening or what really exists.
Mary didn't believe that her cousin could be mean until the cousin opened Mary's eyes by scratching and biting her.
John's eyes were opened up to the world of nature when he visited his grandfather's farm.
Something that makes you understand the truth.
Pam's first visit to school was a real eye-opener.
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Source: A Dictionary of American Idioms