“A Sound of Thunder” is a science fiction story about a man named Eckels who hires a time travel company to take him on a hunting expedition in the age of the dinosaurs. The theme is that little things can make a big difference.
This story reminds us of an idea called the “Butterfly Effect.” The Butterfly Effect states that a very small event can have large unintended consequences. In the story, Ecklels is warned never to step off the path, because they can only hunt animals that are already about to be killed if nature runs its course.
It floats six inches above the earth. Doesn't touch so much as one grass blade, flower, or tree. It's an antigravity metal. Its purpose is to keep you from touching this world of the past in any way.
This little bit of foreshadowing, the insistence that he stay on the path, is directly related to the theme. When you step off the path, something happens that you do not intend. You can affect the entire course of the future, just by stepping on a butterfly.
"Not a little thing like that! Not a butterfly!" cried Eckels.
By stepping on the butterfly, Eckels altered the path of history such that the spelling on the sign was changed and a new man was elected president.
We probably won’t go time traveling in our lifetime, but this is still a relevant theme. Sometimes a little thing can make a big difference. One small choice, although it may seem minor, can affect not only the course of your future but others as well.