Our kids use Teaching Textbooks, a computer program that teaches mathematics. The only stupid part of the whole program is that you have to insert a CD to make it go, and getting a replacement CD costs $15.
David lost his math CD for a couple of weeks. He got further and further behind, and when urged he would go look through his room again, but couldn’t find it. Finally, Jacinta explained the deal to him:
In just a little bit, I’m going to come look through your room. If I find that CD, I’m going to charge you $15, which is the cost of replacing the CD. But if you can find it now, before I get there, then I won’t charge you anything.
And by golly, he was back in fifteen minutes with that CD!