My son's favourite word seems to be "BUT ..." when I'm trying to explain about something he maybe shouldn't be doing :-) Also when a kid says "that's not fair" they normally mean "I'm not getting exactly what I want" which is not necessarily a good definition of fairness. With children you can see the raw routes of behaviour that then stays with many adults in one form or another, it's pretty interesting (makes me look at my own behaviour for example).