My writing clients love this exercise. A lexicon is an inventory of words you can draw from, either to use right in your copy, or to spark ideas for plot, back story and description. You can do this for a theme of your book or for a particular scene. While you are busying your internal editor with making lists, your mind is free to spark ideas off the words. For more information and examples see: http://debmcleod.com/take-the-lexicon-challenge.