如果某人非常高兴，欣喜若狂就可以用 over the moon 这个短语，比喻一个人欣喜至极甚至都可以飞到月球上去。
If someone says you know your onions, it means you are experienced in something or you know a lot about a particular subject.
In English, if you take something with a pinch of salt, it means you don't believe it completely.
如果一件事情并不复杂，很容易理解或完成，那么就可以说 - 'it's not rocket science' 。
