Once upon a time there was a person at a table – let us call him Dave. Or Tony. Or Michael. Or George. Or Francis. Or Taqui. On the table were several piles of nuts, neatly tipped out from their bags, ready for a dinner party or some such delight.
There was also a second person who strolled past, examined the table and remarked that one of the nuts was off; further examination of this person by Dave (or George) resulted in a conclusion that said nut was in the middle pile somewhere. No need to throw the whole pile away, said the second person. Just root through until you find the nut, crack it with the nutcrackers to confirm that is the nut (the kernel will be twisted) and throw that away.
So Dave (or was it Tony?) set steadily to work on the pile with the nutcrackers.
It took some while, and the nut pile kept rolling around as Michael (or Dave) dug around in it looking for likely-looking nuts, and the second person wandered past and remarked on the risk of cross-infection, and picked up some nuts which had rolled away, checked them and ate one or two of them.
Eventually, some hours and a bad wrist later, Francis (or Tony) was left with a pile of cracked nuts and no sign of the bad kernel.
The second person came past again, ate several of these cracked nuts and remarked, between scoffings, that the bad nut had probably rolled away at some point, due to the untidy digging which George (or Francis) had been doing in the pile, and was now in one of the adjacent piles. It might, reckoned the second person, be advisable to go through those too, just to be on the safe side.
Later on in the party, now sans nuts, Taqui discovered that the second person must be an expert on nut conditions, because they sold nutcrackers for a living.
Meanwhile a nice jolly tune to offset the nuts.
(Tom Lehrer singing about sending the marines would probably make the same point.)