The most challenging designs often lead to the most rewarding results.

知识的每一次进步都会带来新的问题和新的困难。

马跑不起来都是因为太瘦；人不能扬眉吐气就是因为贫穷。

It is not the strength of the body that counts, but the strength of the spirit.

购买旧书，它们的词与新词相同，也可以去图书馆。