The first important time travel novel was Mark Twain's "A Connecticut Yankee in King Arthur's Court". The most famous is H. G. Wells's 1895 novel "The Time Machine". Well's book uses a machine that allows an operator to travel to an exact time. Twain's time traveler is struck in the head and wakes up in the past. The term ""time machine"" was invented by Wells. Now it is the name for any vehicle that can take a rider to another time. Ray Bradbury's 1952 short story called "A Sound of Thunder" is a more recent and very famous example of this genre. Time travel stories can be complicated. They have logical problems such as the grandfather paradox. Time travel is a popular subject in modern science fiction, in print, movies, and television.