Since its release 30 years ago, "Back to the Future" has been everyone's favorite time-travel movie. It's remained a must-see long enough for Marty McFly's own kids to enjoy it.

Even so, there's much you may not know about the beloved sci-fi comedy, from the unused ideas that popped up in other films, to why there has yet to (thankfully) be a reboot. To celebrate Back to the Future Day (October 21), here are 30 things you need to know about Marty McFly's first trip through time.

"Back to the Future 30th Anniversary Trilogy" is available to own now on Blu-ray & DVD.

1. Director Robert Zemeckis and co-screenwriter Bob Gale (pictured above) tried for years to create a time-travel story. The key came in 1980, when Gale was looking over his father's high school yearbook and wondered whether he and his father would have been friends if they'd both been teenagers at the same time.

2. Zemeckis and Gale took their idea to Steven Spielberg, who'd already made three movies with them ("I Wanna Hold Your Hand," "1941," and "Used Cars"). Spielberg liked the idea, but the pair held off, fearing that they'd get a reputation in Hollywood as filmmakers who only got work because of their relationship with Spielberg.

3. According to Gale, the pair pitched the script 40 times without success. Disney rejected the idea as too Oedipal. Columbia, on the other hand, felt the film wasn't sexy enough. (This was the era of "Porky's" and the teen sex comedy.) Only after Zemeckis had a hit on his own, his 1984 work-for-hire "Romancing the Stone," did the director have the clout to get "Back to the Future" made -- at Universal, with Spielberg producing.

4. Then-Universal chief Sid Sheinberg wanted the heroine's mother renamed Lorraine, after his wife, "Jaws" star Lorraine Gary. He also wanted the film's title changed to "Spaceman from Pluto," reportedly believing that no film with the word "future" in the title could be a hit. Zemeckis and Gale accepted the first request but rejected the second.

5. Marty McFly was named after a production assistant Zemeckis and Gale had known on the set of "Used Cars."

6. Bullying villain Biff Tannen (Thomas F. Wilson, pictured) was named for Ned Tanen, a studio executive Zemeckis and Gale had clashed with during the making of "Used Cars."

7. To play Marty, the filmmakers wanted Michael J. Fox, but he was unavailable due to his commitment to his hit NBC sitcom, "Family Ties." Instead, they hired Eric Stoltz, fresh from his star-making dramatic performance in "Mask." As the shoot progressed, however, it became clear to the filmmakers that Stoltz wasn't working out. His performance was too "heavy" and lacked the comic energy they were looking for. Five weeks into the shoot, Zemeckis made the painful decision to fire the actor and replace him with Fox, who had arranged to shoot "Family Ties" during the day and "Back to the Future" at night. You can see some of the extant footage of Stoltz as Marty, along with the filmmakers discussing his firing, in this video.

8.Crispin Glover, who played Marty's father, is actually three years younger than Fox.

9.Christopher Lloyd landed the role of Marty's inventor pal, Doc Brown, beating out his "Buckaroo Banzai" co-star John Lithgow, as well as Dudley Moore and Jeff Goldblum.

10. Einstein, Doc's dog and the time machine's first test pilot, was originally written as a chimp.

11. Lea Thompson was cast as Marty's mother because the filmmakers liked her chemistry with Stoltz in the movie "The Wild Life."

12. Yes, that's Billy Zane (far left), in his first feature role, as one of teenage Biff's thug pals.

13. Melora Hardin, best known for playing Jan on NBC's "The Office," was originally cast as Marty's girlfriend, Jennifer. It would have been a big break for her, but before she'd shot a single scene, Stoltz was fired, and she was fired too because she was so much taller than Fox. Claudia Wells was hired in her place.

14. The famous Hill Valley clock tower, so pivotal to the plot, stands on Courthouse Square, a set on the Universal backlot that's been used in such famous films as "To Kill a Mockingbird" and the Spielberg-produced "Gremlins." After its use in the "BTTF" trilogy, it popped up again on TV's "Buffy the Vampire Slayer" and in the movie "Bruce Almighty."

15. Originally, Gale's time machine was a stationary box -- a refrigerator, in fact. To harness the power needed to make it travel through time, there was to be a scene where the fridge (with Marty inside), was taken to a nuclear test site in Nevada, where Doc Brown would somehow capture the energy from an atomic explosion. Zemeckis ultimately rejected this idea, fearing that impressionable kids would accidentally lock themselves in refrigerators and suffocate. But producer Spielberg liked the fridge-nuking idea enough to use it 23 years later in "Indiana Jones and the Kingdom of the Crystal Skull."

16. Instead, Zemeckis came up with the idea of a mobile time machine, which led to the idea of using a DeLorean. The gull-wing-door car, already something of a time capsule joke even in 1985, was ideal, Zemeckis figured, because it could easily be mistaken for a flying saucer.

17. The filmmakers had a product-placement deal with Pepsi (there's even a Pepsi Free joke written into the script). The cola bottler objected to a similar joke about Tab (a product of rival soda maker Coca-Cola), but the filmmakers refused to cut it.

18. For Marty's all-important rock 'n' roll performance at the school dance, Fox learned to mimic the guitar moves with his hands, but his showboating solo was dubbed by guitarist Tim May. His vocals were dubbed by singer Mark Campbell of Jack Mack and the Heart Attack.

19. Remarkably, there are only 32 special-effects shots in the movie.

20. "Back to the Future" cost $19 million to make, including the $3 million spent on re-shooting the Stoltz footage.

21. Zemeckis was worried that the movie's box office would suffer because Fox was unavailable to promote it, having to be in London shooting a "Family Ties" special.

22. In fact, the film earned $211 million and became the top-grossing movie of 1985.

23. The film was No. 1 on the box office charts for 11 of 12 weeks throughout the summer of 1985. (It was knocked out of the top spot once, by "National Lampoon's European Vacation," but then it returned.)

24. "Future" was nominated for four Oscars. It won the prize for Best Sound Effects Editing. It was also nominated for Best Original Screenplay, Best Sound Mixing, and Best Original Song, for Huey Lewis' "The Power of Love."

25. President Ronald Reagan was a fan of the film, even appreciating Doc's joke at his expense. Six months after the film's release, during the 1986 State of the Union address, Reagan quoted Doc's famous line: "Where we're going, we don't need roads."

26. Claudia Wells dropped out of the sequels, reportedly to take care of her cancer-stricken mother, which is why she was replaced with Elisabeth Shue.

27. Crispin Glover also didn't appear in the sequels because of a salary dispute. Glover claims he was being given less than half of what the other principals were being offered. He also has said he believes his notoriously demented guest appearance on "Late Night with David Letterman" scared the filmmakers away. Nonetheless, he eventually reconciled with Zemeckis and played Grendel in the director's motion-capture epic, "Beowulf."

28. For his stand-up comedy performances, Tom Wilson composed a song that answers all the annoying questions people have asked him whenever they recognize him as Biff.

29. Zemeckis and Gale are preparing a stage musical version of "Back to the Future," due to premiere in London later this year.

30. As far as a film reboot is concerned, however, Zemeckis and Gale have said it will happen over their dead bodies. They mean that literally, as they made sure their Universal contract stipulated that no one could remake the film while either of them is still alive.