Mystery Science Khan Academy Theater
I first saw this video in this post which gives some background.
Don't Use Khan Academy without Watching this First - EdTech Researcher - Education Week
The author says Khan Academy is good for teaching procedures, but not the thinking behind them. He quotes Khan in the video saying, "In your own time, think about why these rules apply."
His comment is, "For many math teachers, the most important work to do is to get kids to think about why the rules apply, to help them derive them where applicable, and to help them contextualize them when derivations are impractical."