Group By Time Stamp

Hi,

I have a table Answers with fields answer_id integer,time_stamp timestamp.

How can I create a query that gives a sum of answer_id=ID ,but in every
 2 hours.

For example:

answer_sum   time_stamp

10           2002-03-24 20-22
12           2002-03-24 22-24
20           2002-03-25 00-02
4            2002-03-24 04-06

..............

and so on.