Why does every great religon have a flood story?

There is a logical explanation to this already easy question. The reason every great religion has a flood story is because it is true. It is just seen differently by different cultures. The cultures form a story to explain the occurance and the people carry that story into its culture. Noah had three sons and 3 daughter in laws, each telling the story in his or her own version, and since they all have a version, each of thier off spring hears his or her own parents version of the story.