Given the question: Information:  - A hobby is a regular activity that is done for enjoyment, typically during one's leisure time. Hobbies can include collecting themed items and objects, engaging in creative and artistic pursuits, playing sports, or pursuing other amusements. A list of hobbies is lengthy and always changing as interests and fashions change. By continually participating in a particular hobby, one can acquire substantial skill and knowledge in that area. Engagement in hobbies has increased since the late nineteenth century as workers have more leisure time and advancing production and technology have provided more support for leisure activities. As some hobbies have become less popular, like stamp collecting, others have been created following technological advances, like video games.  - Model building as a hobby involves the creation of models either from kits or from materials and components acquired by the builder . Categories of modelling include : Scale model building Live steam models Matchstick models Military models Model aircraft Model cars Model commercial vehicles Model construction vehicles Building models Architectural models Model figures Model military vehicles Rail transport modelling Model rockets Model ships Freelance model Cardboard engineering Firearm models ( See Airsoft guns ) Gundam Models  - The hobby of collecting includes seeking, locating, acquiring, organizing, cataloging, displaying, storing, and maintaining whatever items are of interest to the individual collector. The scope of collecting is unlimited: "If something exists, somebody somewhere collects them."  - Leisure has been defined as a quality of experience or as free time.  - A video game is an electronic game that involves human or animal interaction with a user interface to generate visual feedback on a video device such as a TV screen or computer monitor. The word "video" in "video game" traditionally referred to a raster display device, but as of the 2000s, it implies any type of display device that can produce two- or three-dimensional images. Some theorists categorize video games as an art form, but this designation is controversial.  - Stamp collecting is the collecting of postage stamps and related objects. It is related to philately which is the study of stamps. It has been one of the world's most popular hobbies since the late nineteenth century with the rapid growth of the postal service, but is now in decline with the rise of the internet which has sharply reduced the number of postal items arriving in households. Stamp collecting proved to be an almost perfect hobby for collectors because there was a never ending stream of new stamps as each country sought to advertise its distinctiveness through its stamps. Because some stamps became rare, a thriving international trade in stamps was created. While stamp collectors are of all ages, it has been particularly popular hobby among children, many of whom continued the hobby as adults.    What is the relationship between 'model building' and 'activity'?
The answer is:
subclass of