I can't find the video, but apparently at some canneries there used to be a person who would very rapidly hit the tops of cans with a metal rod and listen to th

I can't find the video, but apparently at some canneries there used to be a person who would very rapidly hit the tops of cans with a metal rod and listen to the noise that it made. "Bad" cans sounded different and would be removed from the line. After years of practice these workers were inhumanly fast at the job.