Kids and Cryptology

Often times children come up with their own words and different variations of what they are taught.  Some even create their own languages and sometimes secret codes.  I know this is really prevalent in twins who are very close, they come up with their own form of communication.  However, it is a form of cryptology in the fact that it secret to all those who did not create it, i.e. adults.  These languages and codes hardly ever stand the test of time though.