...75th B'day - The Celebration Experimentation: The gang persuade Sheldon to celebrate his birthday and throw him a party with some special guests, including Stephen Hawking. (S9 Ep17/24) [AD,S]
...75th B'day - The Troll Manifestation: Leonard and Sheldon are forced to defend themselves against negative comments from an online critic. Stephen Hawking guest stars. (S8 Ep14/24) [AD,S]
...75th B'day - The Relationship Diremption: Sheldon starts to wonder whether he's wasting his time with his study of String Theory. Featuring a vocal cameo by Stephen Hawking. (S7 Ep20/24) [AD,S]
...75th B'day - The Extract Obliteration: Sheldon is excited when Stephen Hawking agrees to play an online word game with him. Penny signs up for a class at community college. (S6 Ep6/24) [AD,S]
...75th B'day - The Hawking Excitation: In the first of a stack of episodes celebrating Stephen Hawking's 75th birthday, the physicist comes to lecture at the university. (S5 Ep21/24) [AD,S]