Why SUM(null) is not 0 in Oracle?

Question

It would be appreciated explaining the internal functionality of SUM function in Oracle, when encountering null values:
The result of

select sum(null)          


        

Answer 1

This is incorrect: The sum of 0 + null + 1 + 2 is 3; select 0 + null + 1 + 2 total from dual;

Result is null! Similar statements give result null if any operand is null.