After the demise of his father, Gohan reflects on the power he wields and so devotes himself to scholarly research on the source of that strength, ki. Gohan begins researching the martial arts of the past, organizing them in his book Groundbreaking Science in Age 804. The book quietly gains popularity and one of the articles, "Katchin Can Be Cut!", has an immense influence on the birth of the Swordsman class, as it leads to the creation of Trunks and Goten's Kikoukenjutsu Sword School in the year following the book's release. It also leads to Krillin and Tien Shinhan starting up their own training schools in Age 821, the New Turtle School and the New Crane School, respectively. As a result, by Age 1000, many Earthlings have taken up advanced martial arts and understand ki control.