Teenage-Tom chuckled, as he lectured, "Quid pro quo means >this for that<. The phrase is used to signify an exchange of goods, services - or in our case - favours. Originally, Potion Masters used it when they had to substitute one potion or ingredient for another of similar effect. That was common practice when their patients were allergic or a certain ingredient was not available. Over time, the meaning of the phrase was expanded to include all types of exchanges. That brings us to the here and now."