Because every “<em>x</em>” value has two <em>“y”</em> values.
Step-by-step explanation:
In the graph of a function every value of <em>x</em> has one and only one value of <em>y</em>. So, if we draw a straight line which is parallel to <em>y-axis</em> and it cuts the graph in only one point, this graph will correspond to a function.