A vertical line drawn across the graph of a relation cuts it twice. What does that show?

1. A function gives exactly one output value for each input value. 2. A vertical line collects every point of the graph that shares one input value. 3. Two crossings therefore mean that one input value has two different outputs. 4. That breaks the definition, so the relation is not a function. _Source: Siyavula Mathematics Grade 12 (Everything Maths, CC BY 4.0), Chapter 2: Functions_