A Math Magic Trick
This trick is an extraordinary introduction to the power of algebraic thinking, and a reminder of what makes algebra so awe-inspiring. When I was in middle school, I used to make up A video Launch for this lesson is available in two parts. PDF of the lesson is below the videos.
When using as a class launch, stop at 2:45 and let the class come up try to find their own counterexamples.
The solution to the magic trick.