This learning resource focuses on the distinction between relations and functions, specifically using the vertical line test (VLT) to identify whether a given graph represents a function. The document outlines the definition of a function as a scenario where every input correlates with exactly one output, and provides illustrative examples, such as a vending machine for functions and a broken machine for non-functions. The VLT is a graphical method used to determine the nature of the relationship depicted by a graph: if a vertical line crosses the graph more than once, it indicates that the relationship is not a function. The activity includes practice graphs where students can apply the VLT, making it interactive and promoting understanding. Finally, there is a reflection section where students consider the implications of the vertical line crossing the graph in two places.