I found a new Kindle PaperWhite has an awesome feature. It seems to be very useful for cultivating my vocabulary. I used to write down unknown words to a notebook with sentences where the word appeared. Kindle's Vocabulary Builder, which Amazon calls it, gives me the same sort of word book for free. It's created automatically within the device and whenever I look into a word during reading, it collects the word together with the sentences around which the word locates. There are already over 500 words in my device. Turning over such words vacantly will be nice way of passing time when I'm so tired that I cannot read any book.