Each concept car is a kind of work of art. The appearance should deliver aesthetic pleasure and convey the spirit of the brand. [2] Concept cars often offer revolutionary solutions in the design or engine design. Some use non-traditional, exotic or expensive materials ranging from paper to carbon fiber and unusual alloys. Others have an unusual exterior, for example, a gull-wing door, 3 or 6 (or more) wheels, unusual features that are not in the production cars. Due to many impractical or unprofitable findings, concept cars often remain mock-ups or even computer drawings. Other, more viable models can become fully functional, with the engine, transmission, etc. Most concepts never turn into the final product. Inactive layouts are usually made of wax, clay, metal, fiberglass, plastic or various combinations of all of the above.