The English language is a strange affair at times. Meaning is usually gathered from the words we use in the literal sense; the words and the order they are placed in a sentence tell you what the sentence as a whole means. But we don’t talk and write literally, neither do we hear or read, and consequently understand, literally.
I take it you are catching my drift so far? (I’m not really taking anything or expecting you to catch a drift, but I presume you know what I mean.)
We know what a word or phrase means because of the context in which it is used. Take food for instance; we use olive oil and vegetable oil, and we eat cheese sandwiches and we know what these are. They are familiar to us. One word tells you what the connection is to the other word; vegetable-oil, oil derived from vegetable products, olive-oil, oil derived from olives, cheese-sandwich, a sandwich containing cheese. Easy isn’t it?
What if you throw a rogue string of words into the equation? Finger sandwich, baby oil, gravy boat. Would someone from another culture think we ate sandwiches with fingers in, or that we made oil from babies, or that we would make a boat from gravy?
We would not drizzle baby oil on our salads and we would not go to sea in a boat made from gravy, neither would we keep gravy in a boat (not the nautical kind, anyway), just as we would not be eating sandwiches with fingers in. They are not literal pairs of words. But you know what these phrases mean and so do I; we understand what they mean instinctively but it is an instinct that is learned.
Making sense of our language
Speech, reading, and writing are not naturally learned skills. They have to be taught specifically and so does the understanding of these skills. If we weren’t taught the understanding we would take a string of words in the literal sense and not make sense of our own language. We grow up taking it for granted that we can communicate with each other by means of the spoken word as well as the written word.
It is use of phrases that don’t make literal sense that livens up our language and makes it the rich form of communication that it is.