Rule:  A function is a relation in which NO two ordered pairs 
have the same first coordinate.  (It can, however, have two 
ordered pairs with the same second coordinate.)

{(3, 4), (5, 9), (9, 9), (2, 3)}

IS A FUNCTION because none of the four ordered pairs have
the same first coordinate. (It is OK that the middle two
ordered pairs have the same second coordinate).


 {(0, 0), (0, 1), (1, 4), (2, 4)} 

IS NOT A FUNCTION because the first two ordered pairs 
have the same first coordinate, namely 0.


{(2, 1), (4, 5), (8, 4), (1, 0)} 

IS A FUNCTION because none of the four ordered pairs have
the same first coordinates. 


{(8, 3), (8, 0), (7, 7), (4, 7)}

IS NOT A FUNCTION because the first two ordered pairs 
have the same first coordinate, namely 8.

Edwin