My wife was particularly cruel to me last weekend. She pulled out about a half dozen old puzzles and wanted the family put them together. This little entertainment was originally for the kids, but soon I was roped into putting them together by myself. The puzzles were old, and some of them were missing pieces. But they are fun, and a nice distraction during this time of social distancing. But the missing pieces got me thinking. When does a puzzle become useless? Putting them away, my wife marked on the boxes the missing pieces. -2, -4, and so on. The largest was a 100 pieces, and that one, was ironically, complete. But when does a puzzle become useless? I’ve known people to toss…