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If we've got the picture of a critter (i.e. the graph), there's an easy way to tell if it's a function or not.

It's called THE VERTICAL LINE TEST:

If you can draw a vertical line anywhere on
 a graph so that it hits the graph in more
than one spot, then the graph is NOT a function.

 Check out Standard Parabola Guy:

graph of a standard parabola graph of a standard parabola showing that a vertical line does not go through its graph more than once

No matter where we drop a vertical line, it only hits the parabola in one spot.

So, Standard Parabola Guy is a function!

f( x ) = x^2  ...  a number ( x ) goes in  ...  and its square spits out