When I was about 14, I learned a new language, one absolutely guaranteed to infuriate anyone who couldn't speak it: egg language. Egg language has a noble history, having apparently been devised in the early 20th century by suffragettes (for some reason it's easier for women to learn). The rules are that you insert an "egg" in front of every sounding vowel (and "y" where it's used like a vowel), so my first sentence would read as follows:
Wheggen eggI weggas eggabeggout feggorteggeen, eggI leggearned egga neggew leggangueggueggage, weggone eggabseggolegguteleggy gueggaregganteggeed teggo egginfeggureggieggate egganeggyweggone wheggo ceggould neggot speggeak eggit: eggegg leggangueggueggage.
My sister Claire and I learned it from a sort-of friend of mine, K, who was rather an annoying sort of girl, a game player, a would be mysterious "witch", a girl fond of affecting to walk about everywhere in bare feet, and as it turned out a plain old backstabber; and we used to speak it in any situation where, essentially, we wanted to discuss something and not be understood by adults and/or the person we were talking about. A plain old backstabber's charter, you might say, except that anyone overhearing us talk in egg language would know with absolute certainty that there were terrible secrets afoot or, worse, that they were being laughed at.
I can still speak it without even thinking. So deeply has it entrenched itself in my brain that I awoke at 3am recently with the firm belief that I needed to start reading the news in egg language for egg-speakers worldwide, and that I must communicate this brilliant idea to my sister with all due dispatch.