What’s the first thing that comes into your mind when you see these expressions?
– kick the bucket
– be barking up the wrong tree
– a storm in a teacup
– strike while the iron is hot
– have egg on your face
Your answer is probably that they are all idioms: groups of words that not only have a meaning that is different from the individual words, but also a meaning that is often difficult or impossible to guess from the individual words. If someone is barking up the wrong tree, they have the wrong idea about how to get or achieve something; it has nothing to do with – or is unlikely to have anything to do with – dogs or trees. If you have egg on your face, you might need a handkerchief, but it’s more likely that you are embarrassed or feel stupid because something you have tried to do has gone wrong. These expressions are also good examples of the commonly-held view that idioms tend to be very vivid and colourful expressions.
Less obvious perhaps, but the answer, in fact, is the same: they are all idioms. Is the meaning of these expressions very different from the individual words? Not to any great extent. Is the meaning difficult or impossible to guess? Not particularly. Are they vivid and colourful expressions? Certainly not. So, why are they idioms?