If an unverified gossip account on Instagram posts that your favorite celebrity couple just broke up, you might take that rumor with a grain of salt. In other words, you’ll exercise a healthy bit of skepticism and wait for more evidence.

Though no literal salt is involved, it was when the phrase was first mentioned (that we know of) in ancient Rome. In his Natural History, written around 77 CE, Pliny the Elder recounted the story of how Pompey—best known for warring with Julius Caesar—found directions for the concoction that Mithridates VI used to inoculate himself against certain poisons. Mithridates VI famously ingested small doses of poison to build up his immunity, but according to Pliny, the recipe called for other ingredients, too: dried nuts, figs, and rue leaves. Everything should be minced together and taken after having added a grain of salt: addito salis grano.

It’s not totally clear how the phrase ended up with its modern meaning after that. According to Michael Quinion’s blog World Wide Words, some people who read Pliny’s Natural History later on may have mistaken his mention of salt as a figurative warning. As in: “Be skeptical about this recipe, since I’m not sold on its efficacy and you might accidentally poison yourself to death,” or something to that effect. But without any evidence that other ancient Romans used grain of salt as an idiom, it seems more likely that salt was part of the actual recipe. It’s also possible that the idea of using salt to make poison easier to swallow just seemed like an apt description for adding a little skepticism when consuming questionable information.

In any case, grain of salt showed up again in John Trapp’s 1647 Commentary on the Old and New Testaments, but didn’t really catch on until the 20th century. As HowStuffWorks reports, the literary journal The Athenaeum mentioned it in a 1908 issue that read, “Our reasons for not accepting the author’s pictures of early Ireland without many grains of salt … ” By that point, the idiom was presumably common enough for readers to understand its meaning.

But considering the large gaps in the history of the phrase, this rundown can’t exactly be called a comprehensive origin story. In other words: take it with a grain of salt.

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