Most spin-offs aren't that popular, see: Joey, That 80s Show, and The Brady Brides.
But a lot of our favorite shows actually are spin-offs, you just didn't know it.
1. Saved By The Bell
You can thank a bad Disney Channel series called Good Morning, Miss Bliss for your favorite gang of high school students. The original series still featured the characters (and actors) of Zack, Screesh, Lisa, and Mr. Belding, but the show focused on a character named Miss Bliss. After 13 episodes, it was clear that Miss Bliss was no good, so they re-worked the show and added Slater, Kelly, and Jesse.
2. The Muppet Show
No, we're not talking about Sesame Street here. Kermit originally appeared on Sam and Friends, a live-action/puppet show which aired in Washington, D.C. in 1955. In 1969, Sesame Street premiered and five years after that, The Muppet Show came along.
3. Mork & Mindy
The alien sitcom was actually a spin-off from Happy Days of all shows. Mork from Ork (Robin Williams) comes to Earth and tries to abduct Richie Cunningham. Of course, the Fonze wasn't about to let this happen. The episode's original ending is that it was all a dream, thus bringing Happy Days back to reality. However, Mork was SO popular among fans that the episode was re-edited so that Mork erased everyone's memory. This way, a spin-off could happen because he was, in fact, a "real" thing.
4. Family Matters
Not many people have heard of Perfect Strangers, but almost everyone has heard of Steve Urkel. The suspender-wearing, "Did I do that?"-saying, and always hilarious Urkel can thank Harriette Winslow for his fame. Winslow appeared in the third and fourth seasons of Perfect Strangers as an elevator operator. Family Matters was supposed to follow Harriette's family, but once Urkle appeared halfway through the first season, things shifted to focus on him.
5. The Facts of Life
Before we loved Edna Garrett as the house mother in The Facts Of Life, she was housekeeping for the Drummonds on Diff'rent Strokes. Edna appeared on Diffr'ent Strokes for the first and second season. The spin-off ended up being more popular than the original, lasting a total of nine seasons on air.
One of the longest-running primetime dramas on television actually spun-off a different long-running primetime drama. NCIS can thank JAG for giving them their start. The characters from NCIS show up in two episodes of JAG in the eighth season.
7. The Jeffersons
Before they moved to the deluxe apartment in the sky, the Jeffersons lived next door to the Bunkers in All In The Family. Considered the most lovable racist on TV (is...that a thing?), Archie Bunker struggles when the Jeffersons move in next door, as they are the first African-American family in the neighborhood. "They're wonderful people, they're lovely people...but they are also colored people." Joke's on you, Archie. The Jeffersons ran three seasons longer than All In The Family.
8. The Simpsons
I'm sure die-hard Simpsons fans will know this, but the longest running American sitcom was rooted in a variety show, called The Tracy Ullman Show. The family would appear in short, animated segments for the first three seasons of the show, and eventually found themselves on primetime television. The Simpsons went on to become one of the most watched and most beloved TV shows of all times.