Why do books make this sound when a grouping of pages are bent?

Do u want me bent over for you or do u prefer me from the front?

I only bent over to get my books

Here is your daily dose of puffy nipples

Bent Over on the Couch

Kisankanna Bent Over

Bent over against the wall

My roommate ate my ass in the shower, I bent all the way over

Lulu Chu’s tentacle adventure

Just a bit more before I get back to my books

Bent over in public just waiting for you to give it a taste [wbo

Needed to get back to my books but couldnt focus