When Eckels returns from the world of dinosaurs, what is different from the past?
Eckels is the main character in Ray Bradbury's "A Sound of Thunder". He has paid a lot of money to a company, Time Safari Inc., to be sent back into the past so that he may hunt a dinosaur. The company has gone to great lengths to ensure that the past is not changed in any way as it is thought that even a minor change in the past could result in changes to the present. Eckels panics when he see the T. Rex and steps off the levitating path that the company has provided. He crushes a butterfly with his shoe. When he returns to the present, at first things appear to be the same, except for a faint, odd smell. He notices that a clerk in the office looks somehow different, but he can't pinpoint the difference. Eckels then sees that the signs in the office contain misspellings--but in this new "present", the signs are spelled the way they should be. The clerk, with whom he had had a discussion about the election scheduled to take place, now has the polar opposite opinion about who should be elected.
These small differences are noticed only by those who traveled back in time with Eckels as the current world diverged from theirs as soon as Eckels killed the butterfly. This story is the source of the term "butterfly effect" to mean a very small, seemingly insignificant event that has unforeseen results.