Knowledge

Knowledge goes in cycles. Something is invented and soon everyone knows what it is. Then it goes out of fashion and we all seem to forget how it works. Then it becomes junk. A decade or two later someone re-discovers it. Painfully a group of enthusiasts re-learn the 'lost' knowledge, marveling at what they find. Then it (whatever the object is) becomes 'historic' and valuable and the knowledge of it becomes 'lore', highly prized. We do the same with people. We finally realize how much the grandparents or great grandparents had to share only after they're dead.