as changeable as the moon字面意思是“像月亮一样多变”，月亮经常改变形状、亮度等，所以这个表达被用来比喻“反复无常、变化无常”。

【例句】
What can I do with her? She is as changeable as the moon.
她反复无常，我真拿她没办法。

I really don't understand why she is as changeable as the moon.
我真搞不懂，她为什么总是反复无常。