There was once a factory that specialised in armour. They made leather jerkins, full plate suits, greaves, helmets, anything that would stop an arrow or a sword.
One day, an order arrived from a foreign kingdom. It was a big contract, to outfit an entire army with chain-mail leggings. The factory owner was delighted, and immediately took the design specifications down to the factory floor to begin production.
Several days later, a second message arrived. Due to various circumstances, the design of the leggings had changed. The new standardised national military uniform required that the hems be lowered by several centimetres.
The factory owner grumbled about the loss of time and money involved in changing the design, but there was nothing he could do. He took the new plans down to the foreman.
Several days later, another new design arrived. The nation's uniform requirements had changed again, and the hems must be raised, even higher than the original design.
This went on for several weeks. Every few days, there was a change of plan, and the leggings had to be changed. Sometimes the hems went up, sometimes they went down, but every change meant a loss of money. Finally one day, the factory owner called the foreman up to his office, and asked him if there was any way to stop the appalling wastage.
“Well,” said the foreman “it might be that the changes are gradually settling down, and will eventually lead to a stable set of leggings. If so, we could extrapolate from what we already know to find the ultimate design, and start producing it now, knowing that it's what they'll eventually ask for.”
The factory owner agreed this was a good plan.
“On the other hand,” continued the foreman, “it might be that the changes will never settle down to any final form, in which case there's nothing much we can do.”
This prospect depressed the owner, and he demanded to know whether there was any way to tell which situation they faced.
“Oh, certainly.” said the foreman, “There's a simple way to tell.”
He paused.
“It's called the Wire Trousers Hem Test for Uniform Convergence.”