In this paper we explore the idea that learning science involves appropriation of the school-science language and how it manifests in the classroom. This is done through an analysis of peer group discussions in Swedish secondary schools; discussions that served both as an arena for learning and as a research tool. In this arena, the students are offered opportunities of communicating, evaluating and arguing knowledge claims. The analysis focuses on the intersection between social languages (colloquial and scientific), and epistemological/conceptual aspects of biological evolution. We explore how words (especially Vygotsky's meaning and sense of words) and semantic patterns manifest in the students' talk. Specifications are made step by step in negotiations, and the groups of students talk more and more in line with school-science language. We understand this to rely on the establishment of an arena where technical terms and scientific models may be introduced, negotiated, and made sense of, in particular in relation to personal and everyday experiences. The students use an inter-language where colloquial expressions serve as an asset in sense-making. (Contains 1 table.)