It's hard to make a successful network television show. It's even harder to make a successful spin-off of a successful series. Audiences have certain expectations and, when it's time to follow another character or two elsewhere, it can get tricky. But CBS hit it out of the park with "Young Sheldon," which focused on young Sheldon Cooper as he grew up in Texas long before the events of "The Big Bang Theory,” itself one of the most successful sitcoms of the 2000s.

Because "Young Sheldon" takes place decades before the events of the show that inspired it, it was difficult to get "The Big Bang Theory" characters to appear. However, the show's producers managed to sneak Kaley Cuoco in during the show's third season for a sneak cameo. Cuoco, of course, played Penny on The Big Bang Theory for its entire 12 season run.serving as the anchor of the ensemble.

However, when it came time to sneak into Young Sheldon, it was all about Cuoco's voice, not her actually appearing on screen. The moment came in the episode "Teen Soup and a Little Ball of Fib", in which Sheldon (Iain Armitage) pretends to be sick to avoid taking a swimming test at school. Cuoco ended up providing the voice of the pool water. In a 2020 interview with TVLineSeries co-creator Steve Molaro explained how it all came together:

“It was absolutely Kayleigh's voice like pool water. I needed someone to do the voice. I knew I wanted it to be feminine and a little bit sexy, and I said, “Well, duh! "Caley is perfect for this."

So how did Molaro sell this bizarre role to Cuoco? After all, Cuoco has stayed pretty busy since The Big Bang Theory went off the air. voicing the title character in Harley Quinn and "The Flight Attendant," among other projects. Even so, it was easy for the actor:

"I said, 'Hey, I've got a weird pitch for you: Sheldon has a nightmare and he has to go into this pool and he talks to the water in the pool.' Do you want to be the voice of the water in the pool? She just laughed and said, "Absolutely, I do."

Although the role was small and relatively odd, it was also important. Cuoco became the first former "Big Bang Theory" star to lend her talents to the spin-off since Jim Parsons, who served as the show's executive producer in addition to serving as narrator. To help close out Young Sheldon, Parsons reprized his role as Sheldon in the flesh alongside Mayim Bialik's Amy in the final season. That aside, Cuoco's voice acting was a nice touch for fans.

What's interesting is that CBS didn't promote Cuoco's cameo before the episode premiered. Didn't the network want to highlight the Big Bang Theory star's return to the universe? Maybe, but as Molaro explained in the same interview, everyone involved in making the show agreed that the moment was all the more impactful because it was kept secret:

"Sometimes I feel a little weird about things being over-promoted, and as wonderful as it was, I don't know that it should be something that comes up in promotions. (Caley) wanted to keep it under wraps (however) so if you figured it out, it was a little Easter egg for you, so we were all on the same page about it."

The series ran for a total of seven seasons, with the seventh season of "Young Sheldon" wrapping things up in 2024. However, the larger Big Bang Theory universe lives on another spin-off in the form of Georgie and Mandy's First Marriage. Whether or not Cuoco or any of the other Big Bang Theory stars will appear on the show remains to be seen.

Young Sheldon is now streaming on Netflix and Max, or you can grab the complete series on DVD from Amazon.