The sitcom “I Dream of Jeannie” may have lasted only five seasons in the 1960s, but the show remains immensely popular all these decades later.

The program followed the relationship between astronaut Major Tony Nelson (played by Larry Hagman) and a 2,000-year-old genie named Jeannie (played, of course, by Barbara Eden). For 139 episodes, the two shared a chemistry that made America fall in love with them.

The show got an extra push because it debuted at a time that space exploration was beginning to spark real interest across the country.

That said, here are five facts you likely never knew  — and will never forget — about the much beloved “I Dream of Jeannie.”

1.) The first season had a different theme song. The tune of the show’s theme song is so memorable that it was featured in 1986’s immensely popular “Ferris Bueller’s Day Off.”

However, the theme song, called “Jeannie” (what else?), was not actually implemented until the second season. The animated intro featuring Jeannie and Tony in color on an island was used for seasons two through five. In season one, the show was still in black and white (yes, it’s true — it was that long ago).

Definitely a little bit different and not quite as memorable — hence why the first one didn’t last.

2.) Eden wasn’t a fan of the Jeannie-Tony marriage. It was no secret in the first four seasons that Tony was interested in Jeannie — so the writers decided to take that relationship further by marrying off the two characters in the fifth season.

That also ended up as the last season of the show.

On the “Today” show in 2015, Eden told the world how she really felt about the writers’ decision: “It ruined the show,” she said of the Jeannie and Tony marriage. “Because [Jeannie] wasn’t human … She thought she was, and [Tony] knew she wasn’t … I think it broke credibility.”

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3.) Eden was pregnant in the first season. During the first season of the show, Jeannie was a little more covered up than she was for the rest of the show. Why, you ask? It’s because Eden was pregnant while shooting the first 10 episodes of the show.

The producers decided to hide her protruding belly instead of finding a new actress to take on the part. Her son, Matthew Ansara, was born on Aug. 29, 1965. After that, the outfits became a tad bit more revealing.

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4.) Hagman learned from a security guard that the show was canceled. One of the sitcom’s lead actors was somehow a little out-of-the-loop on some big news.

Larry Hagman went on a vacation to South America after the fifth season ended — and when he came back to the United States, he went to the studio to grab something from his dressing room.

When Hagman got to the security gate and wanted to go inside, a guard had to break the news to him. While Hagman said in a 2004 interview that he was not surprised the show ended — the way he found out was “real Hollywood.”

5.) Phil Spector himself made a guest appearance. Before he went to prison for second-degree murder, Spector was a very well-known record producer. During the third season of “I Dream of Jeannie,” he made an appearance in the episode “Jeannie, the Hip Hippie” when Jeannie wanted a big-name exec to listen to her rock group.

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Spector even made fun of himself in the episode. During a phone call, he can be heard saying, “The bridge is very weak. The tempo’s awful. The lyric stinks. I think you got another smash.”

He then proceeded to order 100,000 copies of the record in question.

Tom Joyce is a freelance writer from the South Shore of Massachusetts. He covers sports, pop culture, and politics and has contributed to The Federalist, Newsday, ESPN, and other outlets.