Underlining is a small promise to the future. It says: when you return, start here. The page is the same, but you are not, and the line you chose will meet you in a new way. This is the gift of rereading—finding that you left yourself a map, and that it still leads somewhere worth going.
You do not need to mark much. One line is enough to make the rest of the paragraph glow. One word is enough to hold the shape of a thought. Over time, these small choices form a kind of ledger: what stayed with you, what you wanted to keep, what you were ready to carry forward.
Today, open a book you have already read. Find the quiet signposts you left. Let them guide you through a familiar landscape until something new appears. The mind learns in layers, and the earlier layer is not wasted when a later one settles over it. They hold each other in place.
The margin is not a performance. It is a private room where you learn to listen to yourself. Leave another mark. You may need it again, and even if you do not, the act of choosing will teach you what you value now.