Premack Principle: This is a principle of operant conditioning originally identified by David Premack in 1965. According to this principle, some behavior that happens reliably (or without interference by a researcher), can be used as a reinforcer for a behavior that occurs less reliably.
For example, most children like to watch television--this is a behavior that happens reliably (they learn to like TV all on their own and it is something they will do willingly without any interference from their parents)--and parents often use this behavior to reinforce something children like to do less such as washing dishes. So, some parents might condition children to wash dishes by rewarding dish washing with watching television. I'm not saying that is the right thing to do, only that it is an example of the Premack Principle.