Each product starts as a concept, which can take the form of sketches, specifications or constraints that dictate design requirements. A product designer, or usually a team of product designers and engineers, then takes the concept and, using mechanical engineering software, turns the concept into a comprehensive product design. The final design must be manufacturable, that is, be able to meet manufacturing requirements, such as the ability to withstand specified stresses and strains while minimising the final product's weight. This design phase can take several iterations and refining. It can also include a design simulation phase, using software that tests and validates the product design before the final step of manufacturing.