Sara Gilbert stars as Leslie Winkle, a physicist that works at the same Caltech lab as Leonard. She has had an on-and-off again relationship with Leonard and is bitter enemies with Sheldon.
I saw your pupils dilate when you looked at her, which, unless you're a heroin addict, points to sexual attractionLeslie [about Leonard's interest in Penny]
Leslie: Just so we're clear, you understand that me hanging back to practice with you is a pretext for letting you know that I'm sexually available.
Leslie: Yeah. I'm good to go.
Leonard: I thought you weren't interested in me.
Leslie: That was before I saw you handling that beautiful piece of wood between your legs