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In the early seasons of the show around Christmas the gang always talk about the How the Grinch Stole Christmas (2000). Christine Baranski, who play Leonard's mother, plays Martha May Who in it.
Laurie Metcalf, who plays Sheldon's mother Mary Cooper, is the mother of Zoe Perry, who plays Sheldon's mother Mary Cooper on Young Sheldon (2017).
Unlike most TV shows or movies, when characters in the show eat, the prop food is real. Their prop master previously aspired to be a chef and lovingly cooks every meal seen on screen.
Sheldon (Jim Parsons) can be seen wearing various versions of "Flash" t-shirts throughout the series. In the series The Flash (2014), character Cisco Ramon (Carlos Valdes) frequently wears "The Big Bang Theory" based shirts, such as one with Sheldon's catchphrase "Bazinga!"
When Sheldon (Jim Parsons) plays the theremin (which is widely regarded as one of the hardest instruments to play), that is actually Jim Parsons playing it.