When I was young I marveled at my Uncle's ability to know sizes of wrenches. He'd be working away under some farm machine, often a combine harvester, and yell out, "Pass me the five eighths" or "a inch an d a sixteenth". Now, I was in-experienced with the wrench, and often had to try 3 or 4 before one would fit. But not him. He got the right one 95% of the time. For me it was constant experimentation.
Because it worked. He knew the right wrench,