The importance of laying a foundation for algebraic reasoning at a young age is increasingly being emphasized. In this article, the authors report on an activity that elicits in a natural way algebraic strategies that in a later stage of learning algebra are crucial for solving equations. The activity brings about the students' spontaneous use of symbolic notations. It also makes students' reasoning visible both to themselves and to their teacher and helps build students' conceptual understanding and foster productive classroom discussion. This activity involves the work of students aged 11-12, using a hanging mobile as a balance model representing an equation with unknowns.