The concept of "fairness".
If you've got more than one kid, you've heard maybe once or twice the words "that's not fair!" "It's not fair that daddy was born short, either, but I just have to deal with it." is usually my reply. Or that the neighbors have a boat and I don't. Or that my favorite TV show was cancelled.
It's cliche', but "life isn't fair." We know it. We hate it, but we know it. But kids don't quite get it yet. As they get older, it sinks in (and is later followed, in the teen years, by anger and sullen behavior - a causal relationship? Perhaps....). But how do we deal with it now?
Honestly, I don't know. But I've always just told the kids that "fair" does not always mean "equal." Everyone gets what they need, and sometimes your little brother NEEDS that cookie, and you don't. You got that cookie at his age, and he wasn't born yet. So he didn't get that one you got 3 years ago. Now you're even. This has spotty results at best. But I try to not become a slave to the whining. If you give in, that's when the battle is lost. They're going to find out life isn't fair sooner or later. Especially once they see that big screen TV they've always wanted. Life isn't fair. :)