Every language has certain expressions which when translated literally make no sense.  For example:  When we say that it's raining cats and dogs, we mean that it's raining hard.  The meaning of the phrase has nothing to do with cats or dogs.

Look at these other examples of idiom:

What the teacher said went right over my head.
You could have knocked me over with a feather.
That scene in the movie just blew me away.