Fans of Young Sheldon, a spin-off of The Big Bang Theory, have been quietly teasing out Easter eggs and references scattered throughout the show. One of them came as a surprise - a secret cameo by Kaley Cuoco, famous for her role as Penny on The Big Bang Theory. In episode 10 of the third season, Sheldon becomes paranoid about a mandatory school swim test, leading to nightmarish visions of a "disease-infested" pool. He speaks to the pool about his fears; the pool - with a seductive female voice - reassures him of its cleanliness. Just as Sheldon leans to look into the water, a hand materializes and drags him in. Waking in fright, Sheldon yells "Pool monster!", an experience that leads him to fake illness to avoid the swimming test. Few noticed that the voice of the pool came from Kaley Cuoco, making her the first Big Bang Theory co-star to make an appearance in the spin-off. Steve Molaro, the creator, revealed that this cameo was an Easter egg designed for sharp fans to discover.