A scale model is a physical model which is geometrically similar to an object (known as the prototype). Scale models are generally smaller than large prototypes such as vehicles, buildings, or people; but may be larger than small prototypes such as anatomical structures or subatomic particles. Models built to the same scale as the prototype are called mockups. Scale models are used as tools in engineering design and testing, promotion and sales, filmmaking special effects, military strategy, and hobbies such as rail transport modeling, wargaming and racing; and as toys. Model building is also pursued as a hobby for the sake of artisanship. Scale models are constructed of plastic, wood, or metal. They are usually painted with enamel, lacquer, or acrylics. Model prototypes include all types of vehicles (railroad trains, cars, trucks, military vehicles, aircraft, and spacecraft), buildings, people, and science fiction themes (spaceships and robots). Model building Models are built to scale, defined as the ratio of any linear dimension of the model to the equivalent dimension on the full-size subject (called the "prototype"), expressed either as a ratio with a colon (ex. 1:8 scale), or as a fraction with a slash (1/8 scale). This designates that 1 inch (or centimeter) on the model represents 8 such units on the prototype. In English-speaking countries, the scale is sometimes expressed as the number of feet on the prototype corresponding to one inch on the model, e.g. 1:48 scale = "1 inch to 4 feet", 1:96 = "1 inch to 8 feet", etc. Models are obtained by three different means: kit assembly, scratch building, and collecting pre-assembled models. Scratch building is the only option available to structural engineers, and among hobbyists requires the highest level of skill, craftsmanship, and time; scratch builders tend to be the most concerned with accuracy and detail. Kit assembly is done either "out of the box", or with modifications (known as "kitbashing").