In the world of dreams the rules of reality do not necessarily apply. Each dream is unique and it's a direct link to the soul and spirit. Psychology provides its own theories on what dreams mean, but so did tradition and the study of dreams for thousands of years before any relevant science had developed.
Dream Meanings - Hammer
If you are hit with a hammer then you will hear important news, but if you hit someone with a hammer then expect difficulties and mishaps that are solely your own fault.