Because humans think of one thing in terms of another, metaphors have become our guides to the world, the way we point to meaning with words. Asked to explain one metaphor, it's very likely we'll resort to another, and then another, and so on. You get the idea.
But the fact that you do get the idea, drives philosophers nuts.
Can't metaphors be explained?
Well, no. The link between language and meaning has been notoriously hard to fully explain. Originating as sense data in our limbs before being assembled in our minds as a whole, meaning is our encounter with all of the world, all at once. Metaphor is the go between.
After all, you can't singularly experience every photon slanting into our eyes, but you might (picture this), having arrived 10 minutes late to your favorite restaurant and sitting across the linen-draped table from your dinner companion, realize four bites into your tossed penne that the warm sensation isn't steam, and as he raises his gaze just so to meet yours, now is the time.
And hands touch.
You could have led with the perfectly straightforward, "I don't know where to begin" or, worse, "can we talk?" but that would have left him wondering "what is she thinking?", followed by "she was 10 minutes later than normal!"
You? You just had someone very, very important on your mind.
So yes, metaphors make merry. Bad metaphors can even shape the science we get.
But even the perfectly straightforward question, if the evidence from some experimental philosophy and this example are to be believed, will leave you guessing as well. Because when in comes to sense making, words are but one guide, metaphor or not.