SECRET LETTER
Materials:  

Lemon juice, a carrot and paper.

  

Special note:

De-Classified Information
This technique was originally developed by the British Intelligence services and was used during the 'Cold war' to inform its agents of important news and secret meetings. Now the 'Cold war' is over the technique has become de-classified and you can utilise the same method to keep your trusted gang members up-to-date on important matters.
Instructions: Sharpen the end of a carrot to point. Dip in lemon juice and write your letter on normal white paper. Keep dipping the carrot as you write. The writing will remain invisible until the paper is placed near a heat source such as a radiator.