A key returned on time lets another person’s plan stop depending on your memory.

Small unreturned things stay live in somebody else’s head. Until the key, the book, the dish or the drill comes back, the other person has to keep it on a list, wonder whether to ask, and plan around not having it. Returning promptly does not only restore the object; it closes an open loop you did not know you were holding for them. Reliability is mostly about endings.

Think of one borrowed thing in your house and take it back this week without being asked. If you cannot yet, say when you will. The certainty matters more than the item.