It was a real eye-opener.

Meaning

The experience was unexpectedly revealing or insightful.

Examples

• John: That documentary on climate change was intense.
• Alice: I know, <b>it was a real eye-opener</b>. I never realized the scale of the problem.
• John: Me neither. It made me reconsider my lifestyle.
• Alice: Exactly! For me, <b>it was a real eye-opener</b> about how much we all need to change.