Fair use is a fact-intensive policy consideration. There are no absolute rules. You can point to various set of recommendations ("not more than 10%," etc. etc.), but those are guidelines or recommendations, not the law.
Whether or not the unlicensed use of a copyrighted work is fair use depends in every case on the facts of the case. It also depends on the policy considerations at stake.
There are four main factors (not "rules") that courts are required to consider when they evaluate a fair use defense, as spelled out in Â107 of the Copyright Act. They are (this isn't strictly quoting the act):
- the nature of the use, especially whether it is commercial, nonprofit, or educational
- the nature of the original copyrighted work
- the proportion of original work used
- the effect of use on the market/value for the original work.
BUT, while the courts are required, to consider these factors, they do not have to give them equal weight (and, they can consider other factors as well). It depends on the situation. A critical analysis of a copyrighted Haiku or other short poem could probably reproduce the whole poem. "Probably." That wouldn't be true if the critical analysis was an advertisement. Unless maybe if it was an advertisement for an academic literary journal published by a university.
The virtue of fair use is that it appeals to what is fair, not what is technical. But if you want to get technical, fair use is an equitable defense - in other words, an issue of equity, not of law. Look it up. That's why fair use is ambiguous. It's meant to be adapted to each situation as necessary. As for your point about needing a lawyer - tough shit. That's true of nearly every legal issue outside of small claims court. Your issue isn't with the complexity of fair use, your issue is with the fact that copyright infringement makes you liable for a civil lawsuit, not a criminal one, so you don't get a public defender/free lawyer.
How do you explain fair use to children? "Fair use" means you can use copyrighted works in ways that are "fair," whether or not you have permission. What fair means is up to the courts. You can then cite examples.
That doesn't fully explain the law, but thats not the point when trying to educate children about legal issues. It'd be pretty damn easy for the RIAA to say "you can do what's fair." Or, more accurately, "We'll won't sue you if you're fair with your use of our songs." Or, more accurately still, "We'll probably lose if we sue you for fair uses of our songs in ways we don't give you permission to do."