Yellow Warning Triangle In Book

I've created a Book which has 10 chapters/sections. I made some changes to the individual chapters/sections (while not in the Book) and when I open the Book I see warning triangles next to the chapters/sections that i changed. Now I realize it's making me aware that the chapters/sections were changed, but I'm wondering how I get rid of the triangles. What I'm doing is opening the chapters/sections while in the book and then saving them again, however the triangle is still present!

Any help is appreciated! Thanks!