How about just walking outside and rubbing the chalk on the sidewalk like I've been doing for years?
Because one can then write or draw something, not just rub.
And then you have to decide WHAT to write.
Writer's block can then paralyze you, leaving you standing there with chalk in hand and nothing to say.
Your opponents get bored and leave.
The pool room closes, and locks you outside.
Your car keys are inside.
Stuck, you are on the street when gangs come by and beat you and rob you and leave you for dead.
The police, ever creative, use the chalk to draw an outline around your lifeless body, but being untrained, rub only one side messing up the chalk entirely.
No... don't do it... it's too dangerous. The chalk can become damaged by this process. Buy a fancy gadget instead, be safe!