It's the prerogative of prequel series to litter themselves with call-forwards and references to their parent show. Sometimes it's because the events as presented in the original series require expanding upon, but more often it is as a form of friendly wink to the audience who watched the original, a little in-joke.
Young Sheldon is no different in this respect. Detailing the childhood of Sheldon Cooper - a popular subject for jokes in The Big Bang Theory anyway - Young Sheldon isn't afraid to include events purely to hearken back to its parent show. Young Sheldon focusses on how Sheldon Cooper grew to be who fans knew him as in The Big Bang Theory.
10 Sheldon's Early College Days Are Seen
Early on in The Big Bang Theory, Sheldon's lifelong academic brilliance was demonstrated by references to Sheldon attending college at a young age. Particularly so, when he describes the time from the age of seven to leaving for college as "the longest seven years of [his] life."
Although largely used for humour, as Young Sheldon approached that stage of Sheldon's life in A Secret Letter And A Lowly Disc Of Processed Meat. A Secret Letter detailed Sheldon being accepted for college, and Young Sheldon afterwards depicts him attending. This is one of the single most expanded-upon jokes from The Big Bang Theory explored, having a major influence on its storylines.
9 Birds Already Terrify Sheldon
One of Sheldon's more arbitrary character tics on The Big Bang Theory is his fear of birds, despite his interest in various other forms of wildlife - in particular insects. Entire plotlines are on occasion devoted to Sheldon's hatred of them, stemming from a childhood incident.
While the incident isn't shown in Young Sheldon, the audience is still clearly shown manifestations of Sheldon's phobia straight from the pilot. In Training Wheels And An Unleashed Chicken, he shows fear at the chicken of his neighbour and bully, Billy Sparks.
8 Professor Proton's TV Show Appears
Before The Big Bang Theory had Bill Nye guest-star, it introduced its own in-universe equivalent to his show. Helmed by a scientist named Arthur Jeffries, and using the alias 'Professor Proton', the role was by comedian Bob Newhart. Arthur is a recurring guest character on The Big Bang Theory, with both Sheldon and Leonard being major fans of his.
They allegedly have followed Professor Proton since their childhoods, and in Young Sheldon we see that this is the case. Although never thrust in the audience's face, at numerous times in the show Sheldon watches old episodes of Professor Proton's show.
7 Sheldon's Future Workplace Is Foreshadowed
Although their day-to-day work becomes less important as The Big Bang Theory continues, all the main characters share the same workspace. Leonard, Raj, Howard and Sheldon work at the California Institute of Technology (Cal-Tech), one of the most prestigious universities in America.
This is given several nods throughout Young Sheldon. His careers counselor recommends he think about attending, with Sheldon rejecting it. When he attends a talk by Professor Stephen Hawking there in Pasadena, however, Sheldon admits he could happily work there.
6 The Origin Of The Infamous Catchphrase
Although it was quietly dropped by The Big Bang Theory, Sheldon was for a time notorious for his use of the catchphrase 'bazinga!', used to indicate that he was joking or playing a prank on somebody. The catchphrase took on a life of its own outside the show - but often in the context of mocking it.
Young Sheldon shows the origins of this catchphrase when Sheldon decides to try and 'loosen up'. In A Stunted Childhood And A Can Of Fancy Mixed Nuts, he buys a selection of practical jokes made by a company called 'Bazinga Novelties', which proudly proclaims that 'If it's funny, it's Bazinga!'. As he is prone to do, Sheldon apparently took this slogan to heart.
5 Sheldon's Childhood Bully Appears, Naturally
Not even the smallest references are safe from Young Sheldon. Mentioned only twice in its parent show's twelve-season run, a recurring character plagueing Sheldon Cooper is his childhood bully, by the name of Billy Sparks.
Although somewhat unpleasant, Sparks's appearances tend not to reflect his mentions in The Big Bang Theory, with him being less of an omnipresent bullying presence. In particular, he has never shoved a Mexican peso up a young Sheldon's nose, as he is known for in the future.
4 The Hot Beverage Makes An Appearance
It's something of a misapprehension that Sheldon is entirely lacking in social niceties prior to his twelve seasons of character development - it is just that he follows a very narrow set of rules regarding them, and tends not to deviate. One of his notable polite traits is offering people a hot beverage, often to the point of not letting them refuse.
In Body Glitter And A Mall Safety Kit, Sheldon's friend and enemy Paige is upset about her parents' divorce, Sheldon offers to make her one, despite the frequent emnity between them.
3 Sheldon's Hatred Of Geology Explained
One of Sheldon's most notable traits is his distaste for nearly any scientific discipline that isn't theoretical physics. He looks down on his friends for being engineers, biologists, and even experimental physicists, often played for laughs. His most significant hatred, however, is Geology, a hard science that he treats as if it were something for children.
The irrational hatred is somewhat explained in Dolomite, Apple Slices And A Mystery Woman, involving Sheldon's friend Libby in Young Sheldon. Initially, Sheldon is interested when the much older Libby - who he has a crush on - introduces him to the topic. Once he learns, however, that she sees it as 'babysitting', he experiences his first heartbreak, and the association leads him to hate Geology.
2 George And Mary's Marriage Begins To Break Down
One of the recurring points about Sheldon's backstory in The Big Bang Theory is the unhappy marriage between his parents, stated to include infidelity, arguments, and threats of violence. In initial seasons of Young Sheldon, however, the two are at least somewhat content together, and both devoted to raising Sheldon despite the challenges he poses.
Tensions flare occasionally early on, but it is in Sometimes Life Gets Complicated that the audience truly begins to see the cracks forming in George and Mary Cooper's marriage. They have a truly massive fight, leading George to storm off to a bar and begin talking to a recently-divorced neighbour. It's somewhat poignant for audiences, knowing what will happen prior to George's early death.
1 A Montage Shows All Of The Main Characters
For the most part, there are no hackneyed inclusions of other characters risking continuity snarls in Young Sheldon. The future best friends of The Big Bang Theory are spread across America, living entirely separate lives, not interacting with one another.
In a particularly poignant moment in A Swedish Science Thing And The Equation For Toast, Sheldon thinks he'd always be alone. A montage shows each of the main characters of The Big Bang Theory as children, recognizably themselves despite the age difference. Airing directly after the show's finale, it was a touching tribute.