Big Bang Theory's Sheldon was gobsmacked by an out-of-this-world gift.

A napkin which served as the catalyst for one of TV's funniest moments in years is being auctioned off to benefit charity. And odds are, it will make a big bang as far as bidding goes.

In the Dec. 15 holiday episode of CBS' The Big Bang Theory, Penny almost literally knocked Sheldon out of his sneakers by gifting him with a Cheesecake Factory napkin autographed by Leonard Nimoy. Upon learning that the Star Trek icon also wiped his mouth with the napkin, Sheldon effuses, "All I need now is a healthy ovum and I can grow my own Leonard Nimoy!"

Although at the time the scene was shot, Nimoy hadn't actually put his "John Han-Spock" on the prop, the actor has since done so. In fact, in the video below (at the :30 mark), Jim Parsons (Sheldon) tells the story of how the show sent Nimoy two napkins to sign, in case he made a mistake. (An illogical precaution, it turned out.)

The napkin featuring Nimoy's autograph will be auctioned off at The Beit T'Shuvah "Steps to Recovery" Gala — Nimoy's charity of choice — this Sunday at the Beverly Hilton in Los Angeles.

UPDATE: The napkin wound up going for $1,100.