Okay kiddo, you know how we love going to the library and borrowing books to read? Well, sometimes there are people who don't treat the library books nicely and draw or write on them. That's called vandalism.
It's like if you draw on your bedroom wall even though you know it's not allowed. But when you draw on a library book, it can hurt other people because they won't be able to read or enjoy the book as much. And it can also cost the library a lot of money to replace or fix the damaged books.
That's why it's important to always treat library books with respect and take care of them so that others can enjoy them too!