What should NOT be included in comments? (opinion on a dictum by the inventor of Forth)
- by AKE
The often provocative Chuck Moore (inventor of the Forth language) gave the following advice (paraphrasing):
  "Use comments sparingly.  Programs are self-documenting, with a
  modicum of help from mnemonics. Comments should say WHAT the program
  is doing, not HOW."
My question:
Should comments say WHY the program is doing what it is doing?
Update: In addition to the answers below, these two provide additional insight.  
1:  Beginner's guide to writing comments?
2:  http://programmers.stackexchange.com/a/98609/62203