Nacre, the inner layer of seashells, is a laminated nanocomposite consisting of micron sized pseudo hexagonal aragonitic calcium carbonate platelets with about 20 nanometer thick organic layer sandwiched between the platelets. This nanocomposite has been studied extensively as a model system for the design of new biomimetic nanocomposites. The nano and micro architecture of nacre has many features and nuances, which have been attributed as possible reasons for the exceptional mechanical properties. In our work, we have used computational mechanics routes to model and simulate observed macro response, to quantitatively evaluate the contribution of various components of the nano and micro architecture of nacre to the mechanical properties. We also describe our discovery of platelet interlocks and their impact on the mechanical response of nacre. Our experiments on tensile failure and scanning electron microscopy of nacre specimens, and simulations using finite element modeling, indicate that the interlocks function as a physical restraint against free relative movement of platelets. Hence, these interlocking features need to yield/break before the complete transfer of load can occur to an intervening organic. The observed interlocks play a critical role in the mechanical response of nacre. During failure the features observed in the microstructure of nacre, such as relative rotation between platelet layers, platelet penetration, and other geometrical abnormalities such as an elongated side etc., appear not to be accidents of nature; they seem to exist for a purpose. These abnormalities lead to high toughness and strength, which is necessary for protecting the organism within the seashell.