What is understanding?

Understanding, the way I see it, is knowing how something works and being able to explain it. So to me knowing and understanding is the same thing. If you know something, you understand it and if you understand it, you know it. Things get tricky when you have to know if the stuff you know and understand is true or not because information that is not true is pretty much useless. On that note there are things that you probably shouldn't or don't need to understand. I'm not saying the information should be kept hidden but sometimes things are just not worth the time. Which brings up the question how do you know what is worth your time and effort? surely there are things that would never be used but does that make it unnecessary to learn? And how would you measure the level of understanding in something? unlike video games, life does not show you experience or levels although that would be very cool and i would like to see google glasses or something implement that in their product. But back to the question. How would you measure understanding? at what point would you consider it to be understanding? and if you can't measure it, is it real? is there something as too much understanding? maybe there is. you know when someone knows too much and they get really cocky? maybe that's too much understanding there because no one likes a know it all. why us understanding so subjective? as much as subjectivity is nice, it never gets anything done. it's like being politically correct. it just wastes a lot of people's energy. like i dont understand it.