In Outlier: The Story of Success Chapter 2 Section 4, there is a sentence saying
Is the ten-thousand-hour rule a general rule of success? If we scratch below the surface of every great achiever, do we always find the equivalence of the Michigan Computer Center or the hockey all-star team - some sort of special opportunity for practice?
I look it up in LDOCE5 and Merriam-Webster but only find what scratch the surface means. Googling this expression seems only examples rather than a good explanation.