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The blog article 学习什么技术----可以在退休后再做20年？http://blog.sciencenet.cn/home.php?mod=space&uid=451666&do=blog&id=843227  by blogger罗汉江raised an interesting question which I like to address here  based on my own experience. What I have to offer is

1.     A Necessary condition to enable answer to this question.

2.     A discipline/skill independent answer to this question.

First, the necessary condition. To retire without worries, you need to be financially self-sufficient. By this I do not mean you have to be filthy rich. But your savings, investments, and pension should generate enough income (often called unearned income) that at least equal to your last annual salary. In other words, you can  maintain your pre-retirement standard of living forever.  A usual rule  of thumb   is that you should have enough assets 4% of which should equal to your last annual salary.  For example, if your last annual salary is $40,000 then you need $1 million dollars  of asset to retire comfortably. Social security income are not counted here but considered  as a cushion. Here we are counting on 8% investment return with 4% for living and 4% to  compensate for inflation. For more details consult my previous blog articles on  financial independence

Financial Engineering 101  http://blog.sciencenet.cn/blog-1565-542609.html

Financial Engineering(II) Becoming Financially Independent  http://blog.sciencenet.cn/blog-1565-808222.html

Managing Your Financial Life  http://blog.sciencenet.cn/blog-1565-634189.html

Next, we suggest what one can do usefully in retirement regardless of your  occupation , expertise, or interest. In a lifetime, one accumulates skills and life experiences in a field/discipline that  cannot be easily acquired from just reading a book.Such knowledge are often unstated, intuited, and passed on between mentor and disciples.  But such the ingredients are what might  be call  “hidden rules of the game 潛規則”   for successful living and careers.   Such priceless advice and free help are often of great value to youngster starting out in life.  It helps them to avoid waste efforts in blind alleys or  mistakes they regret later in life. Help/advice freely offered with no expectation of payback are the best kind since there are no hidden agenda associated with them. Yet the giver will often be paid back in many unexpected ways. At the same time it allows the giver to  feel  useful and keeps him/her active. My own experience at Tsinghua for the past 13 years

Introduction to CFINS  http://blog.sciencenet.cn/blog-1565-8897.html

On the Tenth Anniversary of CFINS at Tsinghua  http://blog.sciencenet.cn/blog-1565-487201.html

and in writing blogs on Science Net for the past 8 years amply substantiate  my claim  here. I thoroughly enjoyed the past 13 years of my retired life and my second  and third career since 2001. Most important, I am never bored.