One good turn deserves another.

善行应有善报。/ 善有善报。/ 以德报德。

【世博精解】

Turn, 在这里做名词，解释为（帮助或损害别人的）举动，行为：do somebody a good (bad) turn.做一件有利（损）于某人的事。除此以外，turn还可以做动词，比如：turn a somersault翻跟斗；turn somebody against another使某人与另一人作对；turn a street corner沿街角拐弯。

我们再来学习一些关于turn的词组：turn upside down，颠倒，弄乱。Don’t turn upside down our arrangements upside down.别把我们的安排搞乱了；Turn on the tears，一下子流起泪来；On the turn，正在转变中；To the turn of a hair丝毫不差地。

Deserve, to have earned something by good or bad actions or behavior,应受；值得。Deserve punishment ( a reward )应受处罚（奖赏）；deserve to be mentioned值得一提。Deserving该受的；值得…的（of）：an issue deserving of public attention.一个值得大家关注的问题。