During the first two decades of the twenty-first century, writing and reading fan fiction became a prevalent activity worldwide. Demographic data from various depositories revealed that those who read and wrote fan fiction were overwhelmingly young, in their teens and twenties, and female. For example, an analysis published in 2019 by data scientists Cecilia Aragon and Katie Davis of the site fanfiction.net showed that some 60 billion words of contents were added during the previous 20 years by 10 million English-speaking people whose median age was 15½ years. Fan-fiction writers base their work on various internationally popular cultural phenomena such as K-pop, "Star Trek", "Harry Potter", "Doctor Who", and "", known as 'canon', as well as other things they considered important to their lives, like natural disasters. Much of fan fiction concerns the romantic pairing of fictional characters of interest, or 'shipping'. Aragon and Davis argued that writing fan-fiction stories could help young people combat social isolation and hone their writing skills outside of school in an environment of like-minded people where they can receive (anonymous) constructive feedback, what they call 'distributed mentoring'. Informatics specialist Rebecca Black added that fan-fiction writing could also be a useful resource for English-language learners. Indeed, the analysis of Aragon and Davis showed that for every 650 reviews a fan-fiction writer receives, his or her vocabulary improved by one year of age, though this may not generalize to older cohorts. On the other hand, children browsing fan-fiction contents might be exposed to cyber-bullying, crude comments, and other inappropriate materials.