How do you mark up important words and sentences in the book you read?

Hello, bloggers!

How do you mark up important words and sentences in the book you read? Checking by ball point pen, color marker, put tag on sentence head? They can be many ways and style of each of you. My way is more simple without any tools for marking. I just fold edge of pages triangle, It is so called "dog ear." 

I think dog earring is the best way to mark up words and sentences you can do easily through reading. No pen, no marker, no tag piece, but just fold edge of page. Dog earring does't disturb your concentration for reading, nor ask you to hang pen and marker and put them on the part to be marked up precisely, but guide you to be back to the part you like to check again.

Some opinion which against dog earring insists we shouldn't hurt book pages by folding carelessly, and it shorten lifetime of books. Even though, I follow and defend dog earring cause of it shows history of book, and potentially enhance conversation among readers one after another. If I launch a library in a new concept, I'll set a rule there, "dog ear free" for every users. 

If some people famous in writing or book review left dog earring in my library, the books which hurt by them potentially shows high value to read, and to be an asset of my library to call nice user there.