translation.en: You see the dark line on the side and the top. As you turn the pages of the book, the book can actually detect what page you're on, based on the curvature of the pages.
translation.zh: 你看在页边喝页眉都有黑线。你翻页时， 根据书页的弯曲程度，书本实际上能 探测到你在读哪一页。