The fourth wall is the imaginary “wall” at the front of the stage in a traditional three-walled box set in a proscenium theater, through which the audience sees the action in the world of the play. The idea of the fourth wall was made explicit by philosopher and critic Denis Diderot and spread in 19th-century theatre with the advent of theatrical realism, which extended the idea to the imaginary boundary between any fictional work and its audience. The sauna is a place of recreation and pleasure. By joining the sauna session, the participant becomes a contributor to the performance. Consisting of a cylindric shape, the sauna is nothing else than the fourth wall of the stage, where naked people act or do not act in a play without a storyboard. The sauna can be heated up to 80°C, depending on the outside temperatures and the quality of the oven.