Triggers

I'm trying to understand triggers. I have read the documentation in the 
manual as well as the few pages in the Douglas book about the subject, 
but I don't see how to implement a trigger that simply updates a 
'last_edit' date field in my 'persons' table whenever I do an insert or 
update into my 'participants' table; that is a trigger that basically 
does an "UPDATE persons SET 'last_edit' = NOW() WHERE persons.person_id 
= participants.person_fk". Is that even possible?
-- 
Leif Biberg Kristensen | Registered Linux User #338009
http://solumslekt.org/ | Cruising with Gentoo/KDE