Sometimes you know perfectly well how to solve a problem but, in the heat of the moment, your brain gets seized up.
It's not whether or not you can instantly answer a question, it's how you react when you're faced with something that you can't immediately answer.
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Years ago, a lesson I was trying to teach on simplifying algebra expressions blew up in my face in front of a group of thoroughly confused, and somewhat angry, 7th graders. In the aftermath after the bell had rung, I sat on a desk at the front of the room, looking at the problems on the board and wondering what had gone so horribly, horribly wrong. In a moment of clarity, I thought of what algebra must look like to someone who's seeing it for the very first time, and immediately several obvious sticking points jumped out at me. The next day, still gun-shy from the day before, I decided to try to re-teach the lesson, this time adjusting for the sticking points. The students didn't make a sound as I worked my way through to the end of the first problem, and this seriously unnerved me. Fearing the worst, I turned to face my fate. The most vocal of the peeved and perplexed pupils from the day before simply looked at her classmates and said, "I forget. What was confusing about this?"
When teaching beginners, start at the beginning.