1. The opening used a newly recorded version of "Rock Around the Clock," yet the reruns used the original 1954 take.

Bill Haley re-recorded his groundbreaking song in 1973 for the opening credits of seasons one and two. However, when the series entered syndication, it was retitled Happy Days Again and featured the original "Rock Around the Clock." The 1973 version was not released commercially until 2005.

Image: Discogs

2. Harold Gould of 'Rhoda' was the original Mr. Cunningham.

Gould, who is perhaps best known for playing Rhoda's father Martin Morgenstern, took the role of Howard Cunningham in the pilot. He was offered the role again when the series was picked up, but the actor was not willing to back out of a stage commitment he had already booked. Additionally, Susan Neher — not Erin Moran — portrayed Joannie in the original pilot.

3. Ron Howard was cast in 'American Graffiti' because of his work in the 'Happy Days' pilot.

Director George Lucas requested to see the New Family in Town pilot when casting his defining portrait of 1962 youth. American Graffiti became a box office smash in 1973 and earned Oscar nominations. Its blockbuster success convinced ABC to in turn pick up the Happy Days series.

4. In season one, the network said Fonzie could only wear a leather jacket if standing next to a motorcycle.

Oddly, the Fonz sports a windbreaker and penny loafers at the start of the series. The network was a touch afraid of the hoodlum look and re-outfitted Henry Winkler when creator Garry Marshall was away filming another pilot in Hawaii. Clearly, the more square windbreaker look did not fly. Marshall pushed for the black leather, and ABC agreed in the condition Fonzie only wear the jacket when standing by his motorcycle. This is why you always see the bike by the character's side in season one — even indoors.

5. Gavan O'Herlihy played Richie's forgotten older brother Chuck Cunningham — and supposedly left to become a poet.

Yes, there was an older brother, as you can see in the upper right (above the windbreaker). According to the book Happier Days: Paramount Television's Classic Sitcoms, 1974-1984, O'Herlihy, who played the dim Chuck, asked to leave the show after a handful of episodes. He told Marshall he wished to move to Ireland to become a poet. However, you might recognize the actor from Superman III and the Bond film Never Say Never Again. The character of Chuck, a role very briefly filled by Randolph Roberts, soon just sort of vanished from Happy Days.

6. The man who sang the 'Happy Days' theme also sang the 'Batman' and 'Wonder Woman' theme songs.

Ron Hicklin sang the unforgettable "Sunday, Monday, Happy Days…" theme song. The musician led a vocal group, The Ron Hicklin Singers, who can also be heard on the hooky theme songs to Batman, Laverne & Shirley, That Girl, Wonder Woman, Flipper and more! 

Image: Discogs

7. Both Anson Williams and Donny Most scored Billboard Top 100 pop hits with solo songs.

It's only natural that a series so based on rock & roll would have many aspiring musicians. Anson "Potsie" Williams' crooned his single "Deeply," which peaked at No. 93 on the Billboard charts in April '77. That just beat out Donny "Ralph" Most, who landed at No. 97 with his song "All Roads (Lead Back To You)" in 1976.

Image: Discogs

8. Creator Garry Marshall would play the drums on the show.

Even Marshall got into the rhythm by sitting behind a drum kit whenever possible on the series. You can spot him in a couple of episodes.

9. Mork was created because Garry Marshall's son loved 'Star Wars.' Robin Williams was not the first actor cast.

Anson Williams did not have kind words for the script to "My Favorite Orkan." When the cast received the script to the 1978 episode that introduced Mork, they were living. "[It] was like the worst script in the history of Happy Days," Williams said in a recent interview. "It was unreadable, it was so bad." The concept came about after Garry Marshall took his son to see Star Wars. "Can you do a Martian episode?" the boy asked. The actor originally cast as Mork quit a few days into rehearsal. Marshall asked, "Does anyone know a funny Martian?" Someone responded, "There's this guy in my Harvey Lembeck [Comedy Workshop] cast." Penny Marshall vouched for the comedic talent of this unknown stand-up, Robin Williams. Of course, the rest is history.

10. Henry Winkler, Ron Howard and Don Most all provided their characters' voices for the 'Happy Days' cartoon.

The animated The Fonz and the Happy Days Gang ran from 1980 to 1982. Adding voices alongside Howard, Winkler and Most were narrator Wolfman Jack — giving Happy Days another tie to American Graffiti — and former Family Affair star Kathy Garver.

11. Happy Days had a softball team that played in Major League stadiums.

The Happy Days softball squad featured writers, crew and cast members. The roster at times even included Tom Hanks, Robin Williams, Tony Danza, Penny Marshall, and Rob Reiner. The team played before MLB games in historic stadiums such as Wrigley Field, Candlestick Park and Shea. They even traveled to Japan and Germany to play games.

Image: 22 Words

12. Tom Hanks played a black belt karate expert on the show.

Speaking of Hanks, the two time Oscar winner appears in the 1982 episode "A Little Case of Revenge." He appears as a karate master looking to pick a fight with the Fonz, due to a grudge tracing back to the third grade. It was the first time anyone hit Fonzie.