One a day, plus iron

Nobody can do anything without (1) training and/or (2) experience. I think that the longer I go in my particular line of work (Brazil nut marketing), the more I am able to pick up through either (1) the grapevine, (2) research, or (3 and less preferable) the school of hard knocks.

The key here is that when you acquire knowledge the hard way (method 3), that you do not make the same mistake twice. Once is ignorance, which is forgivable. Twice is stupidity. The decision tree for furnace fixing will be ever-expanding.