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Yesterday's Now I Know was also about pigs, leading a few of you to defend the animals as not being truly filfthy.  Turns out, that's right. - Dan
Sweating Like a Pig
In Pulp Fiction, Jules -- the character played by Samuel L. Jackson -- explains to Vincent (John Travolta) why he doesn't eat bacon: "Pigs are filthy animals." 

It turns out that pigs are not really all that dirty, at least not physically. (They still eat anything, including feces.)  The reason why pigs find themselves at home, wallowing and slathered in mud, is not simply a preference of slovenly gluttony. Rather, pigs dive into mud baths in order to cool down.  It turns out that pigs' sweat glands are ineffective at cooling their body temperatures, and therefore, the animals seek out the dirty water.  (Some researchers, however, think that it's the other way around.)

While this makes biological sense, it causes a linguistic problem.  If pigs don't actually sweat, the idiomatic phrase "sweating like a pig" no longer makes any sense -- until you find out that the phrase has nothing to do with pigs at all, but rather, the iron smelting process.

Smelting is the process of producing a metal from its ore -- in this case, iron ore. As part of the process to produce steel, iron ore is smelted into something called "pig iron."  Pig iron is a brittle version of iron which, because of this trait, has limited uses.  But because it is just going to be melted again to produce steel, it does the job.  It gets its name from its traditional shape, as seen below, which resembles vaguely a group of piglets attached to their mother.  (The ornamental shaping of the "piglets" in the picture are atypical, however.)

When pig iron is originally created from iron ore, the smelter needs to heat the ore to extreme temperatures, and then move the liquid metal into the mold.  Until the liquid cools, it can't be safely moved, as the extremely hot metal is liable to spill, burning whatever it comes in contact with.  

How does the smelter know when the metal is cool enough to transport? When the "pigs" "sweat." As the metal cools, the air around it reaches the dew point, causing droplets to form on the metal's surface.

Bonus fact: There is a "sport" known as "extreme ironing," where competitors take ironing boards and clothes to strange, hard to reach places and iron. (Example? Sure.)

Related reading: "Extreme Ironing" by Phil Shaw -- a book about the bonus fact. One review, five stars.

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