Mike and I are creating a new curriculum for home schoolers called "Law for the Little Guy (Bring your Kids up to Legal Snuff) ". Here is the first lesson, on Quid Pro Quo:
First, we explain the definition: In legal usage, quid pro quo indicates that an item or a service has been traded in return for something of value, usually when the propriety or equity of the transaction is in question. For example, under the common law (except in Scotland), a binding contract must involve consideration: that is, the exchange of something of value for something else of economic value. If the exchange appears excessively one sided, courts in some jurisdictions may question whether a quid pro quo did not actually exist and the contract may be voidable. (wikipedia)
Then, we come up with a mnemonic device for the concept:
Hey Homey, what's up bro?
I gave 6+3 to you, and got back nine
That’s called a quid pro quo
I scratched your back, you scratched mine
Finally, to cement the concept, we find a way to help the kids apply this to real life. Mike thought to buy some chicks along with the Mallard ducks we get every year (BEST pets ever, because they swim away in 6 short weeks and you never have to clean up their poop again for as long as you live). So we brought these chicks home and told the kids "this is Quid Pro Quo: we feed them for 56 days, then they feed us".