Summarize the following:
You can't use a pen with a plastic tip to make a pipe because plastic melts and releases toxic smoke when it burns. Only use a pen with a metal tip that can be unscrewed to reduce the odds that you unnecessarily expose yourself to dangerous chemicals when you smoke.  You can use a pen with a metal tip and a plastic case, but it's better if the case is made out of metal as well. The tip of the pen is the small metal cone that surrounds the point when you're writing.

Summary:
Find a metal pen with a removable cartridge and metal tip.