Right now, the #CharlieCharlieChallenge is making the waves among teens worldwide, and seems like a fun, harmless thing to try (and post about on your wall).
But is it really harmless?
While the Physics of it is simple (the pencil movements can certainly be caused by our collective breathing, the same way the Ouija board works on tiny tremors caused by our constantly-moving muscles), the point of 'what's moving it" really isn't the issue.
The more important issue is this: "What are we giving permission to influence us when we play games like this?"
This report might help.