While being one of the most popular languages on the planet and massively dominating the digital space, English is widely regarded as one of the hardest languages in which to achieve fluency. Partly this is because it has a tremendous array of nuance. Partly it is a result of borrowing and stealing words and expressions from all over the world, meaning that there is plenty of inconsistency in how different nouns and verbs work.
However, one of the big issues with mastering the language of Shakespeare is that it is chock-full of colorful sayings and phrases that you really just have to know in order to understand. Obviously nobody is literally saying that a person's eyes are bigger than their stomach when they are unable to finish the food they put on their plate. No one believes there are necessarily actual double-edged swords involved in a situation that has a benefit as well as a downside. We're clearly not suggesting that when you feel envious, that you truly turn green.
All of these are sayings of various kinds: idioms, proverbs and so on, that help to add flair and provide a linguistic shortcut whereby a speaker can be easily understood. Of course, in order for that to work, the audience has to know what they're on about. Let's see how well you know these phrases. It's time to hit the nail on the head!