This article explores the shapes taken by joking among the Trumai Indians and other groups of the Xinguan Indigenous Park (Mato Grosso, Brazil). This social practice often opposes persons who are in open-ended relationships, and can thus be defined as the default mode of relation: while it occurs prototypically between male cross-cousins, it is also common with Indian or non-Indian outsiders. Contrarily, one demonstrates both shame and respect toward real affines. Identifying this system of attitudes gives no account, however, of either the pragmatic properties of joking, nor its specific social efficacy. Joking, in particular, is remarkable by its inescapable ambivalence, both moral and functional. This characteristic is closely linked to the frame of interaction that joking is built upon, which manages, in the same time, to both follow highly conventional patterns and produce deep destabilization. This paper thus tries to explain the paradox of what could be called a predictable uncertainty and convey, partly from my own experience, what it is like to be part of such a play where the opposition between the failure and success of an interaction becomes blurred.