When something is amazing and unbelievable, you can say that "it blows your mind":
Whenever I think about how many people live there, it blows my mind.
It blows my mind how talented Katie is at such a young age.
Sometimes people use this phrase to talk about something that's surprising in a negative way:
It blows my mind how some people can just lie right to your face.
You can also use this expression in the past tense:
A: How was the exhibit?
B: Oh, it absolutely blew my mind!