错误：有什么你最近才意识到的先前都做错了的事情

Since the summer of 2013 (I think), I have been the proud owner of an electric toothbrush. My impression was that you use a electric toothbrush just like you would a regular non-electric toothbrush. You know, like this:See what the lady in the picture is doing? She’s literally brushing her teeth. Turns out, with an electric toothbrush, you’re supposed to position the brush half on your gums and half on your teeth. That way, the bristles can really do some work on the teeth.I found out when two days ago, I complained to my roommate Sam Shih, who just so happens to be the son of a dentist, that my top row of teeth were still yellow after brushing.“Yeah dude, you’re using the toothbrush wrong.” Well, don’t I feel stupid.EDIT: From what I gather from some of the comments here, my roommate, son of dentist that he is, was still wrong. Half tooth / half gum is still a no-no. Apparently, it’s all about the angle. Thanks Brian Paden and all others who pointed this out.