Sheldon introduces to Penny the notion of the assembly line, and then goes on to explain the differences between the moving assembly line and the assembly line. Also mentions the lowering of per-unit costs, or economies of scale.
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Even though his ringtone purchase is a sunk cost, Leonard is unable to rationally select a new ringtone.
Penny starts a business selling hair barrettes and begins discussing her production capacity and revenue from a single day of work.