There’s no doubt that Leonard and Penny were a fan favorite couple on The Big Bang Theory. The two characters had incredible chemistry pretty much from the start of the series. While the two had their first date during the show’s second season, Penny and Leonard’s on-and-off relationship had fans on the edge of their seats throughout many seasons.

Ultimately, Chuck Lorre used the series' 100th episode as an opportunity to completely change Penny and Leonard’s relationship.

Chuck Lorre Took A Major Risk With Penny And Leonard During Episode 100 On TBBT

Fans were devastated when Penny and Leonard broke up shortly after dating. After all, they’d been waiting for years for those two characters to finally get together, only for them to go their separate ways. Luckily, Chuck Lorre used the series' 100th episode as an opportunity to rekindle their relationship.

In “The Recombination Hypothesis,” Leonard envisions what his life could’ve been like if he had asked Penny out the first time he met her. According to Bill Prady, The Big Bang Theory’s 100th episode was meant to be a full-circle moment, taking viewers back to the pilot of the series.

“Chuck had proposed a very big dream sequence episode — possible futures — of what would happen if they married,” Prady told The Hollywood Reporter. “It also had a Rashomon aspect to it because it was his dream and her dream. That was the starting place.”

Ultimately, it is revealed that Penny and Leonard didn’t get back together in real life. However, upon waking up, Leonard decides to ask Penny out on an actual date, giving the fans the reunion they had been waiting for.

“The thought was, if despite what had happened between you and an ex, you had gone out, it had gone badly, it went up and down, if you somehow caught her in that same moment you initially saw — and we can all think about the moment we saw the person we’re with — and if you had that again, it would make you think,” Prady explained. “You get to the point where Leonard stops and thinks, ‘What would happen if I ask her out again?’“

Prady and executive producer Steve Molaro revealed that the concept for this episode was incredibly risky, as the executive producers felt that having most of the episode be a dream was deceiving the audience. Luckily, Lorre and his team were able to convince them to give it a try. While they initially decided to have a commercial cut after Leonard realizes it was all a dream, they ended up not doing it.

“Our feeling was no, he’s got to go [ask Penny out] immediately,” Prady says. “In the same moment that the audience is going, ‘Oh my God, that didn’t happen,” we have to say, ‘Yes, but this is.’ ”

Ultimately, this episode led Penny and Leonard to rekindle their relationship, which would make it strong till the series finale.

Johnny Galecki and Kaley Cuoco Had Great Chemistry Both On Screen And Behind The Scenes

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There’s no doubt that the cast of The Big Bang Theory all had great chemistry with each other. This includes Johnny Galecki and Kaley Cuoco, who portrayed one of the fan-favorite couples in this series. Fans were rooting for Leonard and Penny to get together from episode one. It’s no wonder why. After all, Galecki and Cuoco had great chemistry both on-screen and behind the scenes.

In fact, Cuoco herself has admitted that the reason why Penny and Leonard worked so well together was due to the real-life connection both actors had.

“I think a lot of what made Leonard and Penny work so well was my relationship with Johnny off-camera, which turned into such sarcastic banter that bled into Penny and Leonard,” Cuoco told Vanity Fair. “Johnny and I’s relationship, in a way, was mimicking Penny and Leonard. They were always giving each other s***, and Johnny and I have a similar relationship—which you do after years of being together. We’ve always had each other’s backs, and you can’t lose that. And I do think it happened on-screen. It made our relationship funny and endearing.”

Moreover, Cuoco admitted she had the biggest crush on Galecki back in the day. “I had a very big crush on Johnny early on. I was so not even hiding it. He has such swagger. We were both dating people at the time, but I only had eyes for Johnny. Then, when I found out he had eyes for me too, I was like, Uh‑oh, this is going to be trouble,” she explained.

Ultimately, the two actors began dating in 2008, although they agreed to keep their relationship private at first.

When Galecki and Cuoco broke up in 2010, things got tense on the set of The Big Bang Theory. "When we broke up, obviously it was a little sensitive for a minute," Cuoco explained. Fortunately, the show’s co-creator Chuck Lorre broke the tension by putting Galecki and Cuoco on more scenes together.

Johnny Galecki Originally Didn’t Like the Way Penny and Leonard’s Storyline Ended

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Just like fans wanted, Penny and Leonard got married in Las Vegas in season 9. They even got a do-over wedding in the show’s tenth season. Even though they had their ups and downs, their relationship remained strong until the series' final episode, when it was revealed that Penny and Leonard were expecting a baby.

While fans were ecstatic over the news, some of the actors had their reservations about this plot line. Initially, Johnny Galecki didn’t understand the series finale when he first read the script.

“The pregnancy shocked me. I didn’t get it at first. Molaro and I talked about them maybe not [being pregnant],” he admitted. “The main takeaway is that these friendships will continue; the writers didn’t want someone to move away. We wanted to leave the audience with the idea that these friendships and the love that they have continues.”

Ultimately, the writers decided to go ahead with the pregnancy plotline, even if Galecki wasn’t fully on board with it.

“I wish we had Penny take one more step before we got to the finale. The way the last episodes broke out, we just didn’t get there,” the actor admitted. “But with that said, I do think it was a great landing place for [Penny and Leonard]. We really wanted to honor that relationship, as well as the ‘Our babies will be smart and beautiful’ line from the pilot as a way to tie it back together.”