This paper analyses introductory statistics students' verbal and gestural expressions as they interacted with a dynamic sketch (DS) designed using "Sketchpad" software. The DS involved numeric data points built on the number line whose values changed as the points were dragged along the number line. The study is framed on aggregate reasoning with quantitative data, but it also draws on the Vygotskian social cultural and historical perspective of learning, particularly, how cultural artefacts including language support learning. Three concepts, namely, data distribution, the mean and standard deviation were analyzed in the study. Tasks were designed such that students first talked about their informal understanding of the three concepts before interacting with the DS; then they interacted with the DS and discussed the connections among the three concepts as they dragged the data points along the horizontal axis. Analysis of students' verbal, gestural and other expressions suggest firstly, that the dynamic sketch mediated the informal meaning of data variability for them, particularly the meaning of standard deviation, which most students only recalled through a formula. Secondly, the interactions with the DS drew students' attention away from focusing on formulas and calculations which was clearly evident in their statements before the DS was introduced, to investigating the connections among concepts. These findings support previous studies that have proposed that the use of dynamic computing tools strongly support students' informal understanding of abstract concepts. Implications for teaching and learning introductory statistics concepts at the tertiary level are discussed.