Many times I was sure I understood something by watching it. Then, when it came time to do it, I realized I didn't understand at all.

And it's not just for math. Try it with learning to light a fire without matches. I watched the survival experts on "Alone" fail at that, and fail at building a fireplace, and fail at building bear proof food storage, fail at archery, fail at fishing, and so on. (I'm not claiming to be a survival expert, I wouldn't last a week on that show.)

No work equals no learn.