Terry Russell and Linda McGuigan of the University of Liverpool draw on their classroom research to offer their thoughts on argumentation, an aspect of "working scientifically," with contributions from two recent research collaborators: Janet Hamilton teaching 4-6 year-olds and Joanne Geldard teaching science to 10-11 year-olds. Between them, they offer a developmental perspective on supporting children's capabilities in this area. The developmental perspective outlined in this article confirms the progress children can be helped to make across the 4-11 age range, so that argumentation becomes an invaluable means of extending science knowledge. Rather than a skill to be turned on and off, reasoned discussion is better thought of as something more permanent, habitual, and constantly available--in children's social lives and at home. Insights into a more productive approach to argument may also be appreciated by parents.