Post-it on the book

Why is there a post-it on the book? Actually, those are the words that mother either doesn't know how to pronounce or dunno the meaning. Last time mother had another book to record down the words but it was troublesome to bring two books when reading to Ethan.

Mother needs to stop for a while to search for the word in another book which is not so good. Furthermore mother doesn't want to write down on the book. So at last, mother found the solution by using the post-it. It is good as it can be torn off anytime without destroying the pages of the book.