There is no moment when a borrowed book becomes yours. Nobody hands it over and nobody says keep it. It just stays on the shelf until asking for it back would be odder than having kept it, and then it has changed hands without either of you doing anything at all. Most houses hold a few things this way: a dish left after a party, an umbrella, a jumper lent for one cold evening. Owned by no decision, only by the slow running-out of reasons to mention it. And somewhere, in someone else’s house, a thing of yours is quietly crossing the same line.