An apple a day keeps the doctor away. 一天一苹果，不用请医生。

活学活用：apple 

as like as an apple to an oyster 毫无相同之处 

The rotten apple injures its neighbours. [谚]一只烂, 烂一筐; 一个坏朋友可以影响一群好人。 

Throw away the apple because of the core. 因噎废食。 

upset sb.‘s apple cart 破坏某人计划