The Big Bang Theory may have finished after 12 seasons, but did you spot one of its co-star’s sneaky cameos in Young Sheldon? Chances are: absolutely not.

It turns out that Kaley Cuoco – the actress who starred as Penny in TBBT – secretly featured in the spin-off show, taunting the titular child genius.

In the season three episode Teenager Soup and a Little Ball of Fib, Sheldon (Iain Armitage) is plagued by nightmares about his upcoming swimming test – dreams which feature a sinister and mysterious talking pool.

And, if you haven’t worked it out by now, Cuoco was the voice of that bubbling water.

“That was absolutely the voice of Kaley as pool water,” series co-creator Steve Molaro recently confirmed to TV Line. “I needed someone to do the voice. I knew I wanted it to be female and kinda sexy, and I said, ‘Well, duh! Kaley is perfect for this.’

“I said, ‘Hey, I have a weird pitch for you: Sheldon has a nightmare, and he has to go to this pool, and he speaks to the pool water. Do you want to be the voice of pool water?’”

“She just laughed and said, “Absolutely, I do!”

So, why is this the first time we’ve heard of it? Molaro explains it was supposed to be a subtle surprise for Big Bang fans to discover.

"[Cuoco] wanted to hold it under wraps [as well], so that if you figured it out, that was a little Easter egg for you, so we were all on the same page about that.”

Young Sheldon season three starts on E4 Thursday 5th March at 8:30pm