Sometimes you know you can get more out of something good, and sometime you have to know when to quit.
The best example of when this works and when this doesn't has to be TV spin-offs - because there is no one else on this planet like a TV executive for ruining something good rather than trying something new.
Let's start with the good - because sometimes spin-offs can bring us something truly wonderful.
Happy Days - The Original
Happy Days ran from 1974 - 1984. Even though it presented an idealised vision of life in the mid-50s to mid-60s it gave rise to 2 ever-popular - and one forgettable - spin-offs.
Spin-Off 1 - Laverne and Shirley
Laverne and Shirley ran from 1976 to 1983. It starred Penny Marshall as Laverne and Cindy Williams as Shirley - roommates who worked in the fictitious Milwaukee brewery "Shotz Brewery".
The two lead characters were originally introduced into Happy Days as friends of The Fonz. The timeline of the series is roughly 1959 - 1967. Laverne & Shirley filmed on stage 20 and Happy Days on stage 19. In fact - there was an animated spin-off from Laverne & Shirley creatively called "Laverne and Shirley in The Army" - and there were two - count them - two albums release from Laverne & Shirley as well as "Lenny and the Squigtones".
Spin-Off 2 - Mork & Mindy
Mork & Mindy was where the world fell in love with Robin Williams as Mork - and alien who comes to Earth from the planet Ork in a small, one-man egg-shaped spaceship. The very beautiful Pam Dawber co-starred as Mindy McConnell - his human friend and roommate. The show ran from 1978 - 1982.
So I hear you ask - how the hell can a show about an alien who comes to earth be a spin-off from Happy Days? Good question.
The character of Mork, played by a then-unknown Robin Williams, first appeared in the Happy Days Season 5 episode, "My Favorite Orkan," which was a take on the 1960s sitcom, My Favorite Martian. Williams' character Mork attempts to take Richie Cunningham back to his planet of Ork as a human specimen, but his plan is foiled by Fonzie (though all this turned out to be simply a dream Richie had). The Mork character proved to be popular enough with the audience to go forward with the planned series of his own.
It's the old Dallas cop-out. It was just a dream.
Spin Off 3 - Joanie Loves Chachi
Joanie Loves Chachi is the more sensible of the Happy Days Spin-Offs - following the exploits of Happy Days characters Chachi (played by Scott Caio) and Joanie (Erin Moran) when they move to Chicago and try to make it on their own with a rock band and music career.
The show only lasted two seasons before the characters were "rolled back in" to the Happy Days storyline for that show's final season.