Complexity exists in the world all around us. While it has garnered the focus of scientists in forms such as quantum mechanics and chaos theory, complexity also exists in the every day world. For instance, in order to understand the transmission of a common cold, one needs to understand probabilistic causation, branching causal patterns, and distributed agency. In this paper we argue that students need opportunities to learn the causal concepts related to complexity and we review research in support of this assertion. We introduce a set of materials called "Flow Blocks" that are designed to give children the opportunity to explore complex causal relationships and their analogical relationships to real world systems. We go on to share exploratory research that we conducted using Flow Blocks and to discuss what the finding suggest for learning about the nature of complexity.