This is #12 in the Sweet Valley Twins series and is, in some ways, somewhat disturbing. The twins' father teaches Jessica and Elizabeth a new 'language' (which results in some very annoying space-filling stuff). He gets both of them to promise not to tell anyone else the secret.
This leads to Amy getting mad at Elizabeth and all the Unicorns getting mad at Jessica. It's rather obvious which one of them will break first.
Then Elizabeth finds out that the entire music class plans to embarrass their substitute teacher in front of her evaluator by using the language in class. This shows just how nasty even elementary students can get towards their teachers. That kind of behavior should not be allowed under any circumstances.
The problem for Elizabeth is that if she tells the teacher then she will be breaking her promise. Things work out in a surprising way, though, and the story ends on a satisfactory note.
Young adult Index