First let me say this is one of my favorite days of Physics class!
This file includes a lesson plan, activity worksheet/ writing sheet, as well as a PowerPoint presentation that talks about some of the physics used in the nail balance problem.
I use this the first week of class for student engagement, as the students really enjoy the challenge of this problem. I also use it to model group work, team work, collaboration, lab safety, and scientific problem solving and thinking. This lesson can really be used any time during the course of study. The PowerPoint presentation can be modified to fit your personal needs and would be appropriate for middle or high school grades.
I know your students will enjoy this as much as mine do, and I hope you have fun with it too!