儿子最近开始学加减和进位。由于经常数钱，对 penny 换 dime 是再熟悉不过了，三天时间就学会了“满十向前进一“的规则。让他多做了两天的练习，记住规则，就按部就班地开始学两位数的减法。开始是简单地减法，不用从十位数借。小人儿还自我得意了一番：“ That’s easy, daddy. I can do it by myself 。”

上星期三开始要学从十位数借，老爸给他讲了怎么借，小人儿依样画葫芦，居然把书上的两页练习轻而易举地做完了，让老爸一乐，“猫父无鼠崽嘛”。

谁知第二天，让他一个人做练习题，他哭开了：“ Too hard, daddy. I don’t know how to do it any more. ”

老爸只好放下自己的书，来帮他一把。给他重新讲了一遍“借”的原理，让他做了个 43 减 27 的练习。他倒好，从十位的 4 那里“借”是“借“了，并且把全部都“借“光了：“ Daddy, I don’t think I have enough to do the minus 2. ”

老爸奇怪，这儿子今天怎么不开窍了呢？只好问问他是怎么想的：“ How come there is nothing left after you borrowed 1 from the 4? ”

“ Daddy, I borrowed 4. ”

“ Why should you borrow 4? You only need 1, not 4, to have enough. ”

“ No, I do need 4. 3 plus 4 is 7. So I need borrow 4 to make 7. ”

这下老爸也没词了。只好干巴巴地喝了一口水，理理思路，想想对策。有了！儿子不是对钱很熟悉嘛。

“ You see, you need borrow 4 cents from me. However, I only have dimes. So you have to take 1 dime, not 4 cents, right? ”

终于让他懂了从十位“借”的含义：“ OK, OK, I get it now. ”

。。。。。。。

又作了两天的从十位数“借”一的练习了，还没出错呢，看来这回真懂了。

( 12/2/2006 ）