The most iconic line from the Universal Studios classic horror movie The X-Files poking fun at Frankenstein's use of thunder to add drama to a scene; however, if it wasn't for censorship, this trope wouldn't exist.
Frankenstein was released in 1931, the same year as the other industry-changing horror movie, Dracula. Both films pushed boundaries, and were largely able to do so because they were released during the pre-code era, before the Production Code istration (PCA) was enforcing the "Hays Code" — the Motion Picture Production Code that outlined what was and was not allowed in film. However, once the PCA was formed in the 1930s, battling censorship became much more difficult for studios.
The PCA was responsible for reviewing movies that would be aired in the United States, and it provided certificates of approval, or "seals," to prove that the movie did meet the organization's standards. There was a collective agreement among theaters to only show pictures officially endorsed by the PCA. If a movie failed to obtain such a seal, it would be very difficult to find a theater that would show it. When Dracula were given another widespread theatrical release in 1938 as a double bill, they were subjected to PCA screening. At the time, the PCA determined that some "offensive" material needed to be cut — and cuts were made the the master negative, not individual copies. As a result, the newly-censored version of Frankenstein became the new standard that would be released going forward. One such cut resulted in the iconic "It's alive!" line.
In the original version of Frankenstein, after the unhinged doctor successfully brings his creation to life, he yells "It's alive! It's alive! In the name of God, now I know how it feels like to BE God!" The line was axed by the PCA, who felt its message of "presumption" was offensive to Christians. This had always been a concern for the movie, and in 1931, the studio sidestepped it by including the movie's iconic opening scene in which actor Edward Van Sloan clarifies that the movie is about "a man of science who sought to create a man after his own image without reckoning upon God." The PCA, however, felt that this was not enough and removed Frankenstein's likening himself to God from the movie. In order to hide the cut, thunder is used. For many years, the version of the scene that people saw had the thunder instead of the last half of the line. As a result, what's persisted as a trope is "It's alive!" punctuated with dramatic thunder.
Because censorship cuts were made to the master negative, for most of its history, Frankenstein was viewed in its censored version. The movie was including as part of the 1958 Shock! package of Universal horror movies that the studio sold to television stations for late night programming; the package proved to be a surprise hit, renewing interest in monster horror movies and creating the monster movie fandom that led to publications like Famous Monsters of Filmland. Despite the changing ideas regarding religious themes in movies and what was and was appropriate for viewers, Frankenstein wasn't restored to its original version until an early print was found in the mid-1980s.
