"This is Laverne DeFazio – she's mine!" Fonzie said as he introduced Penny Marshall to the Happy Days universe in the episode "A Date with Fonzie."

The plot of the episode found Fonzie helping Richie to muster the courage to ask out girls by going on a double date with Laverne and Shirley, two cool chicks from Fonzie's little black book.

In his memoir, My Happy Days in Hollywood, Garry Marshall wrote, "There are a few episodes that I look back on as gold because they changed so many actors' lives forever and were remembered by audiences as being extra-special. One of these was 'A Date With Fonzie,' when my sister and Cindy Williams first appeared as Laverne DeFazio and Shirley Feeney."

That first appearance of Laverne and Shirley came about in a rather abrupt, but organic way.

Garry called Penny up one day with an invitation that she couldn't refuse: "Do you want to do a Happy Days? We need two fast girls…. If Cindy wants to do it, she can play opposite Ron. They were good together in American Graffiti. You can play opposite Fonzie."

Penny and Cindy quickly agreed, so that's exactly what wound up happening, and that first appearance of the characters was such a hit that a spin-off series was immediately ordered. It all happened fast for these "fast girls" who were originally meant to be a flash in the pan on Happy Days.

To create the spin-off, Marshall had the concept of two brewery bottle-cappers who are best friends, but he needed to do more to flesh out the characters, and that's when he got inspired to draw upon an electrifying experience he had while dating during his youth.

"To create the characters of Laverne and Shirley, I took pieces from their Happy Days appearances and then took material from two characters I had once seen in Brooklyn," Garry wrote.

He goes on to describe a scene that took place in 1958 after he and some army buddies had returned from serving in Korea. They were out late one night performing at a nightclub.

"We met some girls and took them to a coffee shop at 2 a.m.," Garry said. "Suddenly another girl said something rude to my date. My girl turned to me and said, 'Garry, can you hold my coat?' And then my date beat up the other girl."

Garry said he was wide-eyed, having never seen a fistfight break out between two girls before.

"It fascinated me," Garry said. "The tough-as-nails quality of Laverne and Shirley was based on that single night fight in Brooklyn." On Happy Days, Shirley has a much tougher edge than her Boo Boo Kitty-hugging persona on the spin-off.

When "A Date with Fonzie" first aired in 1975, Penny wrote in her memoir, My Mother Was Nuts, "I was not expecting anything to come of it."

"What happened, of course, is now television history, because Laverne & Shirley ran for eight years," Garry wrote.

"Penny was a girl from the Bronx, who came out West, put a cursive 'L' on her sweater and transformed herself into a Hollywood success story," Penny Marshall’s family once wrote in a tribute, praising her indomitable determination to become a star.

Let's just say that both Penny Marshall and her most famous character Laverne DeFazio are made of tough stuff.