Dry an already dry tree to create a bookmark. The more you dry the tree, the flatter it becomes, making it the perfect way to mark your spot in a book without needing to fold a page or write on it.
What are you bookmarking anyway? Look down and see yourself putting one of the tree bookmarks on a page in your notebook that says "She owes me reeds (しおり)." Oh yeah, you had to make this bookmark in the first place because your friend took your reed bookmarks and didn't give them back.