Because the definition of a romance novel does not limit the types of plot devices, time frames, or locations that can be included, the genre has grown to encompass a wide variety of material and spawned multiple sub-genres. Sub-genres of romance are often closely related to other literature genres, and some books could be considered a romance subgenre novel and another genre novel at the same time. For example, romantic suspense novels are often similar to mysteries, crime fiction and thrillers, and paranormal romances use elements popular in science fiction and fantasy novels.