How can you tell if something is a function or not a function

This characteristic of non-functions was noticed by I-don't-know-who, and was codified in "The Vertical Line Test": Given the graph of a relation, if you can draw a vertical line that crosses the graph in more than one place, then the relation is not a function.

Also, if you can get all the coordinates> (3,5)
3=x value
5=y value

If the x value ever appears more than once then it's not a function
Example (1,5) (2,6) (3,9) (2,4)
Is that a function?
No, because the x value, 2 appears twice.

Example: (4,5) (2,3) (1,5) (3,7)
Is that a function?
Yes, because non of the x values appear more than once, there for the vertical line test will nvr have 2 intersections

Hope this helps!