Draw at different places of the coordinate plane. If one line the graph more than once, the relation is *not* a function. On the contrary, if no vertical line cuts the graph more than once, the relation is a function.

Notice that $ℓ_{3}$ cuts the first graph at two different . Also, $m_{2}$ passes through two different points. Then, neither Relation I nor Relation III is a function. In contrast, any vertical line cuts the graph of Relation II at most once. Thus, Relation II is a function.

- Relation I is not a function. $×$
- Relation II is a function. $✓$
- Relation III is not a function. $×$

Keep in mind that, before stating that a relation is a function, the vertical lines drawn have to cover the entire to ensure that no vertical line cuts the graph more than once.