So there's that outburst, "Holy s**t!" that is best said in a more polite way. Saying "Holy Cow!" is a good substitute. A lot of people say this when in awe of something, or if you drop a hammer on a foot. I find myself saying, "Holy Cow" more than most people.
So here is another explanation I painted for Holy Cow - or Holey Cow, in my Idiom series.
Here are the sketches as I worked on what a "Holy Cow!" really looks like.