"Leonard, Friends are like toilet paper, It's good to have extras under the sink" ― Sheldon to Leonard talking about Frank and Alicia.
Frank and Alicia are some friends of Sheldon, who are only mentioned in "The Fermentation Bifurcation". Sheldon mentions them as alternatives to his regular friends whom he figuratively likes to keep under his sink and have to maintain a little mystery with his regular friends.
Trivia
- Sheldon mentioned to Leonard that he plays Elder Sign with them as an alternative to D&D with Leonard, leading to believe that he regularly plays with them.