Knowledge learned before but won't use again is usefulness. However, when you found you have to use some knowledge, you will feel those are not enough for use and regret that why you didn't learn more or better before. 

     Most of textbooks used in primary school, secondary school have threw away after the final exams, but I still keep some books at home because I think they maybe useful in some future days. I'm always not willing to throw them away, but out of space, I have to do so. 

     Today, I'm regretting I threw some math's text books away because I forgot some important formulas when I have to use. Or maybe, I have to learn things with heart and then I won't forget those again.

  書到用時方恨少！

2003

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