The literature on argumentation and education often conveys that children’s argumentation skills are not well developed; therefore, it would be difficult to find argumentation in small children, as well as in primary school classrooms. However, studies focusing on argumentation in less formal contexts show that there is no need to depart from such a negative stance. If children are given room to pursue their lines of thought, they often produce sophisticated spontaneous argumentation. In this paper I consider arguments from definition (...) introduced by children as a case in point. To do so, I use a corpus of data, in which small children under the age of six years, discuss with an adult and with peers. Results show two uses of arguments from definition by children: on the one hand, children may introduce a new issue and their standpoint supported by an argument from definition; on the other hand, children may contest or refute an issue that was proposed by an adult and put forward an argument from definition. (shrink)