how to represent what functional dependencies are?
Not sure if this is such a brilliant example, but assume a relation like:

CREATE TABLE OWNERSHIP
( item_id
, ssn
, firstname
,    unique(item_id, ssn) );


Assume the f.d.: 

ssn -&gt; firstname 


For the tuples:

(1, 'X', 'a name')  
(2, 'X', 'a name')


We can see that X in both cases has the name 'a name'.

Another way of thinking about it is: Assume two tuples of R have the same value of A but different values for B. Then by definition B can't be functionally dependent of A.