Three classrooms of Grade 4 students from different schools and diverse backgrounds collaborated in early algebra research to solve a series of linear and quadratic generalizing problems. Results revealed that high- and low-achieving students were able to solve problems of recognized difficulty. We discuss Knowledge Building principles and practices that fostered deep understanding and broad participation. Students used the online Knowledge Building environment Knowledge Forum[R] to conduct their work and we illustrate how Knowledge Forum supported a Knowledge Building culture for mathematical learning and problem solving. Analyses of participation patterns and note content revealed practices consistent with Knowledge Building principles, specifically "democratization of knowledge," with students at all achievement levels participating, and "epistemic agency," with students providing evidence and justification for conjectures and generating multiple solutions to challenging problems. (Contains 2 figures and 11 footnotes.)