Here's a good example of a sudoku technique I use which I haven't seen discussed. If the highlighted square at the bottom was a 6 or a 9, it would make it so the 6s and 9s in the bottom two squares could be switched around and still solve the puzzle. Because a valid sudoku puzzle has only one solution, the highlighted square must be a 4.

Whatever is commonly saved, becomes a unit of mutually understood account, and for reasons of convenience tends to become the media of exchange as well. If you and I know each other and we both SAVE the same thing, I can give you gold (or bitcoin or whatever) rather than giving you dollars which you then have to convert. Thus the key to replacing the dollar is NOT to replace its currency function - it is to replace its saving function. This is why precious metal will be recognized as the money and Bitcoin will not.