Role-playing or roleplaying is the changing of one's behaviour to assume a role, either unconsciously to fill a social role, or consciously to act out an adopted role. While the Oxford English Dictionary offers a definition of role-playing as "the changing of one's behaviour to fulfill a social role", in the field of psychology, the term is used more loosely in four senses: To refer to the playing of roles generally such as in a theatre, or educational setting; To refer to taking a role of a character or person and acting it out with a partner taking someone else's role, often involving different genres of practice; To refer to a wide range of games including role-playing video game (RPG), play-by-mail games and more; To refer specifically to role-playing games. Many children participate in a form of role-playing known as make believe, wherein they adopt certain roles such as doctor and act out those roles in character. Sometimes make believe adopts an oppositional nature, resulting in games such as cops and robbers. Historical re-enactment has been practiced by adults for millennia. The ancient Romans, Han Chinese, and medieval Europeans all enjoyed occasionally organizing events in which everyone pretended to be from an earlier age, and entertainment appears to have been the primary purpose of these activities. Within the 20th century historical re-enactment has often been pursued as a hobby. Improvisational theatre dates back to the Commedia dell'Arte tradition of the 16th century. Modern improvisational theatre began in the classroom with the "theatre games" of Viola Spolin and Keith Johnstone in the 1950s. Viola Spolin, who was one of the founders the famous comedy troupe Second City, insisted that her exercises were games, and that they involved role-playing as early as 1946. She accurately judged role-playing in the theatre as rehearsal and actor training, or the playing of the role of actor versus theatre roles, but many now use her games for fun in their own right.