Young children have difficulties to make connections between objects and events around them. They are not able to think systematically and articulate how things are related; lack of reflection makes this worse. We introduced software called Model-It to address the challenges. A model is a simplified representation of a system that concentrates attention on specific aspects of the system such as more complicated ideas, objects, events, or processes. Constructing, testing, and evaluating models is central to scientists' daily practices. This study, followed a "design-based research" approach, found seventh graders using Model-It were able to create meaningful models of water quality and decomposition after their investigations over three seasons. They performed a variety of valuable modeling practices such as planning, analyzing, synthesizing, evaluating, and publicizing and became more purposeful. This book was written for science education researchers, school science teachers and anybody who are interested in using modelling for facilitating student-centered science learning.