Fickle; volatile; moody;
Moody and unpredictable
The mercurial girl liked chocolate one day and vanilla the next.
MERCUR - IAL
A thermometer changes a lot because it has mercury.
You probably use a lexicon every day. What is it?
Answer:A dictionary. Each time you spell check, you're using a lexicon.
(c) 2014 E. Carruthers