兜兜七岁，时常问一些我在这个年纪问不出的问题，说出一些我即便到现在也说不出的话。
有一次，放学回家的路上，兜兜指着远处的山峰问：”爸爸，我们走这么快，为什么那座山好像没有动呢？”这可真不好回答。我只好粗略地说，因为距离远。”等你上了中学就懂了。”
前些天复活节假期，全家旅行去萨尔茨堡，兜兜问：”路上要多长时间？”我答道：”两个半小时。”兜兜：”那就是一个小时？”孩子，你这是存心捣乱吗？解释清楚后，兜兜说：”要说两个小时半，就好了。”我不得不承认，这是个好主意。
今天，兜兜妈去植物园割了一些野韭菜，给我们做晚饭。做的是什么呢？兜兜妈管它叫韭菜盒子，这是东北话；我管它叫菜角，这是河南话；就是把韭菜和炒鸡蛋剁碎成馅儿，包成大饺子的模样，煎熟即可。
京生得知这韭菜不是超市买的，而是从草地上割来的，就问：”这是草吗？”
我意识到，爱国主义和科普教育的机会来了，于是放下筷子，开始长篇大论：
“这是个好问题。你看，中文的草，菜，药这三个字，都是草字头。很久以前，这三种东西长在一起，长得很像，人们分不出来，饿了不会吃，有病不会治。
“后来，有个叫神农氏的人，中国人哦，很勇敢，冒着生命危险自己去品尝，尝了100种，分出了哪些好吃又健康，就叫做菜；哪些不好吃或者有毒不能吃，就叫做草；哪些能治病，就叫做药。小朋友分不清，在外边玩的时候不能乱吃。
“不光是菜哦，肉也是。我们为什么吃猪肉，牛肉，羊肉，鸡肉，不吃狼肉，狮子肉，大象肉呢？很久以前，人们花了很多时间去试，才挑出了这些好吃的健康的肉；那些我们平常不吃的肉，就是不好吃或者不健康的，小朋友不要乱吃。”
京生想了想说：”那狼，狮子，大象真是很幸运，没被人吃掉，能活的时间长。”
我觉得，儿子这简短一句话的价值，秒杀我前面啰里啰唆说的一切。因为他自己悟出的这个道理，是庄子的观点。庄子讲的故事是这样的：
从前，有个木匠，路过一棵大树，看上去非常粗壮茂密，树龄有几百年了。大家说，这树砍了盖房子多好。木匠却说，这棵树肯定没用；要是有用，早就被砍了。
木匠夜里做梦，这棵树对木匠说：“你凭什么说我没用？我对人来说没有用，但我能保自己的命，这就是最大的用处，连这个都不明白，你才没用呢。”
庄子原话是这么说的：
山木，自寇也；膏火，自煎也。桂可食，故伐之；漆可用，故割之。人皆知有用之用，而莫知无用之用也。
我孤陋寡闻，“无用之用”我到高中才第一次听说，听说的时候还觉得豁然开朗。如今，却被7岁的儿子轻易点破了。
One day, my wife picked some wild garlic chives from the botanic garden and cooked it for dinner. My elder son J asked, ‘is it grass?’
I explained to him the difference between grass and vegetables. ‘Don’t eat anything wild that you don’t know, not only wild vegetables, but also meat. Pork, mutton, chicken are good for our health. That is why we eat them. Why don’t we eat the meat of rats or squirrels? They are not tasty or good to out health’, I talked a little further.
J said, ‘then rats or squirrels are lucky. They can live longer than those eaten by us.’
This reminds me of a Chinese Philosopher, Master Zhuang, who lived 2400 years ago. He once told a story.
Once upon a time, some villagers wanted to cut a big tree so as to build a house. The tree was already hundreds of years old. However, the most experienced carpenter in the viilage said that it was not a good idea because the tree was absolutely useless. If the tree were useful, it would have been cut long time ago and could not have been still living.
In the night, the carpenter had a dream. The tree said to him in the dream, ‘why did you say that I am useless? Well, I am not useful to humans, and this is just why I can survive. I am useful to myself. If you don’t understand this, you are the one who is useless.’
Everyone knows the use of useful, but does anybody know the use of useless?