Lord Foulkes of Cumnock, the former Labour Scotland minister, said the other day of Nigel Farage: "He is like a bull in a china shop and has just come into Scottish politics with flat feet and muddied the water." (The Ukip leader, heckled by protesters in Edinburgh, had been locked in a pub by police before being carted off to safety in the back of a riot van.) Such turns of phrase appear occasionally as fillers in the New Yorker, usually culled from the pages of lesser publications, under the heading: "Block That Metaphor!" It's sound advice, on the whole. There's nothing necessarily wrong with mixed metaphors, if they are well mixed: by flooding his china shop, Foulkes almost comes up with an evocative image, though he rather spoils it with the flat-hooved bull. But they are usually the result of carelessness or overambition, and either way make for baffling reading.

Metaphors don't have to be mixed to be a problem: any figurative expression, if it's overused or used carelessly, can be confusing or off-putting. (And, yes, it's true that all words are figurative, but it's also true that some are more figurative than others.) Wolcott Gibbs, on the staff of the New Yorker from 1927 until his death in 1958, once compiled a set of 10 guidelines for editing fiction. The last was: "Try to preserve the author's style, if he is an author and has a style." The third: "Our writers are full of cliches, just as old barns are full of bats. There is obviously no rule about this, except that anything you suspect of being a cliche undoubtedly is one, and had better be removed." One implication of this is that cliches are fine if you don't notice them; there is such a thing as prose that is too laboriously original. Metaphors, new or shopworn, work when they bring things into sharper focus. If they don't do that, they are best avoided. It's especially annoying when expressions that have a precise literal sense are used in an imprecise figurative way, so that meaning is lost from both. Some examples:

Earthquakes

The epicentre of an earthquake is the point on the Earth's surface directly above the hypocentre, which is the point at which a fault first ruptures. It shouldn't be a fancy way of saying "absolute centre", which there's never any reason to say: "centre" will do just fine. Similarly, the word "shift" almost never needs to be qualified with "seismic". It also probably isn't a good idea to ask someone if the earth moved.

DNA

"DNA" isn't a more scientific way of saying "essence" or "soul" or "core of one's being". DNA is often called the "blueprint" of life; that's a good metaphor. No one would mistake the blueprint of a building for its essence. Come to think of it, no one would try to say a building had an essence. This isn't mere pedantry, there's an ideological problem with it, too. Saying that something is "in someone's DNA" is too often a way of saying there's nothing that can be done about it and pretending that that's a scientific fact.

Catalysts

A catalyst is something that speeds up a chemical reaction but is itself unchanged at the end of the reaction. Someone who sparks a revolution by setting themselves on fire shouldn't be described as a catalyst.

War

Ian Hacking, in The Social Construction of What?, made a "gentle protest" against such labels as "the culture wars" or "the science wars": "Metaphors influence the mind in many unnoticed ways. The willingness to describe fierce disagreement in terms of the metaphors of war makes the very existence of real wars seem more natural, more inevitable, more a part of the human condition. It also betrays us into an insensibility toward the very idea of war, so that we are less prone to be aware of how totally disgusting real wars really are." The "war on drugs" may have begun as a metaphor when Nixon declared it in 1971, but it soon became all too real. More trivially, the title of Martin Amis's essay collection, The War on Cliche, is self-defeating.

Food and recipes

In the words of Alan Partridge, "I promise you tonight we'll have a real half-pound cheeseburger of a show for you. And it's a cheeseburger that contains lots of meaty chat, a salad of wit and a flap of amusing cheese." At least there's no mention of an onion with layers that can be peeled back to reveal more onion.

Journeys

Metaphorical journeys are so commonplace as to be pretty much unavoidable, though that's no excuse for such bureaucratic excrescences as "direction of travel" and "going forward".

The very metaphor itself. "Very" and "itself" don't help. Bang a drum as loudly as you like, it will never sound like an oboe.

The proverbial duck

Proudly owning up to a cliche ("water off the proverbial duck's back") or trying to conceal it ("water off a mallard's back") doesn't make the cliche any less a cliche, it merely draws unwanted attention to it. If it looks like a duck, walks like a duck and quacks like a duck – even if it's a mallard on steroids going forward with layers of onion woven into its very DNA at the epicentre of the war on geese – it's still a duck.