I remembered a day when I was an undergrad psych major at Yale (a day in 1964, if you can believe) when my professor did an experiment in class that stayed with me for all these years.
He had a little mouse and what looked like a Skinner Box – you might remember those from your college years, an enclosure designed to use positive reinforcement – receiving food pellets in a tray – to teach the mouse to perform some action, like pressing a lever.
The prof came into our lecture hall in front of a hundred or more bright-eyed freshman carrying a modified version of the box: It had two separate levers that would each deliver food pellets, and one of the levers was designed to pay off much more frequently than the other.