The Big Bang Theory has done its last experiment, wrapping up after 12 seasons and 279 episodes. The series finale was a sweet celebration of friendship that probably wouldn't have felt earned if the show had ended earlier in its run, when its characters were still single and its humor was a little more biting.

The first half of the two-parter found Sheldon (Jim Parsons) and Amy (Mayim Bialik) finally winning the Nobel Prize for physics, and then Sheldon struggling with the life changes winning was bringing, pushed over the edge by Amy's prize money-induced makeover. In an existential panic, he ran out of the apartment and encountered Penny (Kaley Cuoco) in the now-working elevator. Penny took Sheldon out for a drink, where they reflected on how much their lives have changed for the better since they met. Sheldon accepted the only constant is change. And that's no theory.

The finale jumped forward two months, and opened with the reveal that Penny and Leonard (Johnny Galecki) were expecting, a long-awaited turn of events that people involved in the show had said probably wasn't going to happen. Then the whole gang set out for Sweden to watch Sheldon and Amy (Mayim Bialik) accept their Nobel, with Penny spilling the baby beans on the plane. Sheldon was getting on everyone's nerves with his rudeness and everyone wanted to leave, and even Amy admitted that sometimes even she is hurt by his selfish, inconsiderate behavior. But they ended up sticking it out for the ceremony, where Amy gave a sincere, sort of fourth wall-breaking speech encouraging girls to do science and to not let anyone deny them, and then Sheldon won them back at the ceremony by throwing out his boring 90-minute speech and giving a heartfelt tribute to his friends. He asked them all to stand share in his and Amy's victory, because he couldn't have done it without their love and support. Oh, and Raj (Kunal Nayyar) made friends with Sarah Michelle Gellar. And then the last shot of the series was the gang sitting in the living room, eating Chinese food, while Ed Robertson from the Barnenaked Ladies sang an acoustic version of the theme song on the soundtrack.

TV Guide got on the phone with executive producers Steve Holland and Steven Molaro to talk about ending the iconic series (Holland, who took over as showrunner after Molaro gave up day-to-day operations to run Young Sheldon, did most of the talking). They shed some light on why there are no plans for another spin-off, broke down some of the decisions made in the finale, and previewed what's next for them now that TBBT is over.

Johnny Galecki and Kaley Cuoco,<em> The Big Bang Theory</em>Johnny Galecki and Kaley Cuoco, The Big Bang Theory

You ended with Len and Penny expecting, but not Sheldon and Amy. What went into that decision?
Steve Holland: We already know from Young Sheldon that Sheldon and Amy do have kids. So that didn't seem like a big reveal at the end of our show. And it also felt nice to really honor the Leonard and Penny relationship. The beginning of the whole series was them meeting and their relationship. And really, going back to the line from the pilot, which was "Our babies will be smart and beautiful," it felt like a really great way to sort of honor the beginning of that relationship and the beginning of the show.

How did that Sarah Michelle Gellar cameo come about?
Holland: We were breaking out the scene where knew they were all gonna be on a plane all going to Stockholm and we knew and we knew that Koothrappali didn't have anyone to sit next to him, and [co-creator] Chuck [Lorre] thought that might be a fun opportunity for him to sit next to someone fun. Like someone famous or just someone interesting, we sort of started banding about who that could be, and we've always been huge Buffy fans and then we've referenced Buffy on the show before and it felt like a fun deep pull for them to get to meet someone neat that we also want to meet.

Hahaha, it was just a roundabout way to meet Sarah Michelle Gellar.
Holland:
Exactly. We had to have a whole show and get it to 12 years and then write her into the finale. That was our grand plan.

Did you always know that the elevator would work in the finale?
Holland:
It's interesting, because we know there were sort of wishlists for fan things that they wanted to see in the finale and we tried not to be to beholden to those, but there were a few things that we also really wanted to see in the finale and I think the elevator has been on our list for a long time too. And then it just became a matter of figuring out the best way to do it and honestly earlier on when we were kinda breaking up the finale we had imagined that scene would come in much later. Much closer to the end, but as we were sort of figuring out the finale it just seemed to make emotional sense for Sheldon at that moment and it seemed to be a bigger surprise to sort of come earlier than I think people were expecting.

What do you think the legacy of this show will be?
Holland:
We've been asked that certainly a few times now, but it's so hard to think about what that legacy is. We've been so in the trenches of this season and writing this finale, it's kind of a little bit hard to look past it. Obviously we're aware that the show has had a big impact, and that's something that we're really proud of. Hopefully it's pushed culture a little bit, it's pushed science a little bit, and pushed acceptance a little bit. I just honestly hope personally for the fans that it's a show that they can revisit in the future and it still makes them happy, it makes them laugh.

Steven Holland, Chuck Lorre, Steven Molaro, Johnny Galecki, Kaley Cuoco, Kunal Nayyar, Mayim Bialik, Melissa Rauch, Jim Parsons, Simon Helberg, and Bill PradySteven Holland, Chuck Lorre, Steven Molaro, Johnny Galecki, Kaley Cuoco, Kunal Nayyar, Mayim Bialik, Melissa Rauch, Jim Parsons, Simon Helberg, and Bill Prady

I know Chuck Lorre said a second spin-off was considered, but no idea came together.
Holland:
I don't know if he said it's been considered. I'd say he thinks the question has certainly been asked a lot. I can speak for Steve and I, and I think for Chuck, that this to us has always been an ensemble and it's hard to imagine any of these characters existing outside of this group. Those decisions aren't entirely up to us, but I think that has always been our feeling on it.

So you don't think you would consider another spin-off in the future if the right idea came along, or would you?
Holland:
I mean personally, and who knows, everything could change. But personally, I think this is a wonderful 12-year chapter that we got to do. We got to end as well I think we could have possibly ended it, and I think it's sometimes better to leave that alone.

What's next for you guys, once this all comes to an end?
Steven Molaro:
I will be continuing on Young Sheldon for Season 3 and Steve Holland will be joining me on that.

How do you feel about that, Steve?
Holland: Excited. I love that show. It's gonna be a nice sort of way to transition out of Big Bang and get to do something fun with people that I love.

Any other ideas in development?
Molaro: Running Sheldon is gonna keep our hands full for a while.

The Big Bang Theory is available to stream on CBS All Access.