What moments in the story illustrate the theme of "A sound of thunder?"
This phrase appears twice in the text, and it’s used as a metaphor. (Meaning: it uses one term or image to describe another.) It refers to a noise equal to the most intense thunder you have ever heard. The first time we read it is when the Tyrannosaurus rex crashes onto the scene. Bradbury uses the thunder reference here to let us know that the dinosaur is enormous and that it dominates the landscape. The animal strikes fear into those who see, hear, and smell him, and especially the hunter Eckels.
The second time the phrase appears is in the last line of the story. We know that Travis has a gun. We know that he is upset with the changes that have occurred as a result of the butterfly that Eckels accidentally stepped on, in past time. We are left to conclude that someone was injured or killed by the gun, which at close range would have been quite loud and destructive. It is up to us readers to decide what happens and what the “sound of thunder” indicates here.