Leonard Nimoy became famous and beloved to generations of science fiction fans as Mr. Spock on "Star Trek." But he really won over a certain segment of millennials -- born long after the original series -- by teaming up with the hit TV show "Big Bang Theory." He first "appeared" during a fan-favorite episode from 2008 where the CBS show used a Nimoy-autographed napkin as a plot device.

The napkin appeared in the Christmas episode "The Bath Item Gift Hypothesis." Sheldon Cooper announces that gift-giving is a huge imposition, but his neighbor Penny wins him over by giving him a napkin signed with a "live long and prosper" by Nimoy. Watch the clip below:

In a later episode, Sheldon gets some "Star Trek" collectibles and is haunted by his Spock action figure. The toy was voiced by Nimoy, who was apparently physically on set for the role. 

Nimoy said he came out of retirement to do the show. "The 'Big Bang Theory' has been an ongoing conversation for a long, long time regarding an appearance. Some time ago they asked me if I would provide a napkin that I had used and I did. They used it on the show as a gift to the Sheldon character. It has become one of the most highly talked about and most highly enjoyed shows of all time, I understand. They are a wonderfully talented bunch of people, and it's a smart show and a show full of talent," he said.

"They asked me to appear on the show. For various reasons, a physical appearance didn’t work out, but when they came up with this idea of a voicing of the Spock character with Sheldon being given a 'Star Trek' transporter, the whole idea was wonderful. It was a way for me to deliver a kind of appearance on the show and to work with those very, very talented bunch of people."