I'm sneaky

It's a fine line getting the boys to practice their instruments by having intrinsic motivation and me forcing them to practice. I know from my psych classes that if you're forced or get rewards for doing something, you'll stop doing it when those go away. So I made a chart with Alex's and Nathan's names on it next to each other, and they mark how much they practice for the week. Each day they color in the number of minutes on the graph, and they can see very clearly who is "ahead". I will ask, "I wonder who will practice more today?" Now they are practicing like crazy. Today they decided to not practice, but I convinced Alex to practice one minute, and then told Nathan Alex was one minute ahead, and it was on from there; they've been practicing all morning. I also tell them how excited their teachers will be when they find out how much they've been practicing! They both really want to please their teachers.
Anyway, when I get a good idea I want to write it down so I can use it again later.