The show is centered on five characters: Roommates Leonard Hofstadter and Sheldon Cooper, two physicists who work at the California Institute of Technology (Caltech); Penny, a blonde waitress and aspiring actress who lives across the hall; and Leonard and Sheldon's equally geeky and socially awkward friends and co-workers aerospace engineer Howard Wolowitz and astrophysicist Rajesh Koothrappali. The geekiness and intellect of the four guys is contrasted for comic effect with Penny's social skills and common sense.