I used to be an avid reader on the subject, however with age I have lost some of the finer details. I was thinking back upon what I learned the other day and I was wondering if I had recalled correctly that a particles mass is less than the sum of its parts. Can anyone here tell me if this is true or not? Also are there any theories or explainations as to why this is so? (If it is true of course :) ) Any help would be much appreciated.