Which of the following graphs is not function?

select one:

Important:  characteristic of a "function" is the fact that there may be ONLY ONE y value associated with any given x value.

Look carefully at the 2nd graph.  If we were to draw a vertical line through this graph, it would intersect the graph in 2 places, meaning that for a single x value we have two distinct y-values.  Not acceptable!  

So:  It's the second graph that does not represent a function.