It has long become something of a hobby for fans of Young Sheldon to spot Easter Eggs and references to The Big Bang Theory littered throughout the spin-off. However, few viewers noticed Kaley Cuoco's cameo in one of the scenes.

Episode 10 of season 3, titled Teenager Soup and a Little Ball of Fib, shows young Sheldon paranoid about a "disease-incubating" swimming pool at his school before a mandatory swim test.

His paranoia grows to the point where Sheldon has a nightmare about the pool. In the dream, Sheldon stands next to it and talks to it about all the bacteria swimming around. The pool replies that it is "as clean as Sheldon's daddy's plate after Thanksgiving dinner".

The pool asks Sheldon to come closer to see how clear the water is. As Sheldon reaches the edge, a giant water hand pulls him into the pool.

Sheldon wakes up in horror and screams, "Pool monster!" This nightmare leads Sheldon to fake being sick in order to skip a swimming test.

Few viewers noticed that the seductive voice from the pool was actually that of Kaley Cuoco, who played Penny on The Big Bang Theory. The cameo makes Cuoco the first of Jim Parsons' former co-stars to appear in the spin-off, on which the actor serves as narrator and executive producer.

The co-creator of the show Steve Molaro revealed to TVLine that it was a little Easter Egg for all TBBT fans. The creator said that he wanted the voice of the pool to be "female and kinda sexy" so Cuoco was his first choice. After Moralo pitched his idea to Kaley, she laughed and immediately agreed to voice the pool.

Neither the creator nor the actress wanted this brief cameo to be over-promoted, so they agreed on Cuoco not being mentioned in the credits.

The cameo was left for the wittiest fans to discover. And for some of them, it was a comparatively easy puzzle to crack.

"Only one woman on earth says "Shelden" like that," one of the YouTube comments under the scene clip reads.

TBBT fans have also noted that this nightmare explains why the adult Sheldon "does not float". Leonard says in the original show that Sheldon learned to swim on a floor using the internet, and never tested his skills in actual water.

Well, now we know why.