Question

How do you tell if a graph is a function

Answer 1

You just draw a parallel line to the y-axis. If this line intercepts MORE THAN ONE POINT, you can deduce that the graph is NOT a FUNCTION 

Answer 2

take a vertical line and place it on thegraph. If the graph is a function, then no matter where on the graph you place the vertical line, the graphshould only cross the vertical line once.