Jimmy and I have a battle of wills over the word "sorry".  Boy is he stubborn, and just won't say it!!  And although I try not to let it get to me, it usually does .  Oddly enough, he will often come up to me much later, eg 30 mins to an hour later, and say "sorry Mummy" in a heartmelting way, so I always feel guilty about trying to force him to say it earlier .

I went to a parenting seminar a while ago where they discussed use of "please/thank you/sorry" etc.  According to the presenter, you shouldn't expect your child to truly comprehend the meaning of these words until around age three.  But she said there's nothing wrong with encouraging their use earlier - if nothing else it's good practice for parents .  But I guess what she was trying to say was, because toddlers under age 3 are mainly using the emotional part of their brain rather than the rational part, that it isn't worth having a battle of wills over the use of these words.  That doesn't mean giving in to all their demands, just using distraction etc more - to quickly get their mind off whatever they were fixated on wanting.  So with the cracker example, you could offer him a choice from two other foods you were happy with him eating instead - if you thought he was genuinely hungry - or if not hungry, just move him to another room to play with a toy, something like that. 

The seminar was really interesting by the way - helped me to understand what to expect and tips for managing behaviour at different ages.  They do free/low cost presentations all over NZ - check out the http://brainwave.org.nz/ - brainwave trust website (look at the calender to find upcoming local events) if it sounds like your cup of tea.