This article describes a learning trajectory on expressions and equations based on the sixth-grade Common Core State Standards for Mathematics (NGA/CCSSO, 2010). A classroom teaching experiment using design research (Lamberg & Middleton, 2009) was conducted. The data was analysed using Corbin and Strauss' (2014) Constant Comparative method and Creswell's (2007) data analysis spiral. The realized trajectory that emerged and the design decisions that were made were documented. The findings reveal that student understanding of the changing meaning of letters and variables within a problem context played a central role in student development of conceptual understanding of expressions and equations. Specifically, solving the different problem types involved the following types of thinking: "Label Thinker," "Formulaic Thinker," "Substituter," "Solver," and "Functional Thinker." The letter was interpreted as a label representing a category, a known value, a variable as an unknown value, and a variable as a changing quantity depending on the problem context. Implications for classroom instruction and curriculum development are discussed.