Why brain training is (probably) pernicious hogwash

So-called brain-training tools seem to have exploded in the last few years; one estimate puts it at a $6 billion market by 2020. It's clearly become a major industry, but what's less clear is exactly what it does, and if it even works. The typical procedure seems to be to engage in short games, puzzles and working-memory-type tasks, and these are supposed to produce long term changes in attention, engagement and general fluid intelligence.