The touching 'Big Bang Theory' tribute you may have missed

Warning: The following review is full of spoilers.

The most heart-tugging goodbye to "The Big Bang Theory" didn't happen during the one-hour finale Thursday or the retrospective documentary.

Those who caught the season finale of "Young Sheldon" that aired between those two programs know exactly what I'm talking about.

The spin-off has always had a sadder, more melodramatic tone than its sibling show. The latest episode was the most notable example of its ambitions as the young genius faces the fact that his favorite teacher (Wallace Shawn) is losing his grip with reality and will no longer be able to serve as his mentor.

As 10-year-old Sheldon waits to hear who would receive the year's Nobel Prizes, he starts to cry, convinced that he's doomed to be alone for the rest of his life. But then, with The Supremes' "Someday We'll Be Together" playing in the background, we see a montage of his future friends, each alone in their own way.

The narrator, a grown-up Sheldon, acknowedges that he would turn out to be wrong. Love and acceptance were in his future.

The sweet, unexpected moment served as a sharp contrast to the conclusion of the "Big Bang" one-hour finale, in which Sheldon turns his Nobel Prize acceptance speech into a tribute to his pals as well as Sarah Michelle Gellar, who, in the episode's wittiest joke, winds up as Raj's date to the ceremonies.

It was more than a little incredulous to believe Sheldon would suddenly use his greatest achievement to dine on humble pie.

Worse, it was just plain sappy.

The show did manage to acknowledge the signficance of the moment by breaking some major news (Penny's having a baby!), wrapping up a long-running joke (the elevator finally works!) and giving some recurring characters a chance for one last bow (Brian Posehn's Eeyore-like geologist deserves a sitcom of his own).

The actors also had some nice turns. Watching Kaley Cuoco feast on pickled herring was a reminder of her ability to get laughs simply from her pitch-perfect comic delivery.

But no one could have really expected "Big Bang" to have a five-tissue farewell. Just like Sheldon, the show has always been uncomfortable dealing with emotions. Last week's episode, in which Leonard and his mother hug out their differences was probably its boldest effort.