The author reflects on Robert Fabricant's insight that behavior is a designer's medium, influencing how we shape user actions and impact lives. While commercial pressures often focus on metrics like conversions and revenue, the author argues for a deeper focus on behavior change. Behavior change, drawing from various social sciences, aims to understand and influence human actions. Defining behavior involves observable actions, while change signifies a shift in habits or attitudes. Throughout history, thinkers have explored behavior change as a path to better outcomes for individuals and society. In HCI and Interaction Design, user behavior is inherently central, though its explicit application is a more recent development. Designers frequently use behavior change techniques, but these often fail to create lasting impact. The author proposes embracing behavior as a design material, akin to technology, to create more meaningful products. This involves shaping system behavior to influence user actions, recognizing the unpredictable nature of human agency. Unlike static materials, behavior unfolds over time, requiring designers to sculpt temporal patterns. Duolingo's "streak" feature exemplifies this by using a visual cue to foster a daily learning habit. Ultimately, interaction design should focus on how people and systems behave toward each other, with behavior as its substance. The author criticizes much of current design for prioritizing efficiency over human transformation. Technology should empower users, not confine them to cycles of distraction and dependence. Designers have a responsibility to balance human and business needs, prioritizing human well-being. Applying behavior change effectively remains a challenge for many designers due to its perceived scientific complexity and the difficulty of translating theory into practice. Despite existing models, a lack of standardized heuristics and techniques hinders consistent application. However, consciously treating behavior as a design material offers the potential for deeper, more lasting impact.