Photo: Shenghung Lin [Flickr] Once in a Blue Moon: A neat description of "not very often," it refers to the second full moon within a month - a rare thing indeed. Full moons happen about every 29.5 days, and since a typical month runs between 30 to 31 days, the likelihood of two in a month is slim. But over the course of a century there'll be 41 months with two full moons, so once in a blue moon really means - if you want to get literal - once every 2.4 years. Mad as a Hatter: Today we know enough to keep clear of mercury, but hat makers once used it to make the brims of hats. When absorbed through the skin, it could wreak havoc on the nervous system: tremors, fatigue, not to mention behavioral dysfunction - that is, crazy behavior. Just think of Lewis Carroll's Mad Hatter from Alice's Adventures in Wonderland. Raining Cats and Dogs: In 1600s England it was common practice to discard any waste into the streets - even dead household pets. Once it rained so much that the now-deceased Tabbies and Fidos became buoyant and floated along the streets, thus inspiring writer Richard Brome in 1651 to record, "it shall rain dogs and polecats." Saved by the Bell: Before modern medicine, it was hard to determine if a person was really dead or simply in a really, really deep sleep. As a precaution, the presumed dead were buried with a string that ran from the corpse's finger to a bell. If there was a mistake, the person could twitch the finger and thus be saved from being buried alive. The Acid Test: Gold Rush miners tested possible gold nuggets in acid. Unlike other metals, gold won't corrode in acid, so if the nugget didn't dissolve it passed the acid test and therefore must be pure gold. If a person passes a figurative acid test, they're telling the truth, as opposed to the literal acid test, which would be quite painful, not to mention corrosive. In the Limelight: Theater stages used to be illuminated by heating lime (calcium oxide) until it glowed brightly. Lime has a high melting point, and when heated, gives off a brilliant white light. The light was then focused into a spotlight, so if an actor was in the limelight, he was certainly the center of attention (and probably very hot as well.) Dog Days: The ancient Romans noticed that the Dog Star, Sirius, rose at the same time as the sun on the hottest days of the year, so they made the natural assumption that Sirius in the sky added to the heat of the day. Today it's generally accepted that the "dog days" of summer are July 3 through August 11. But they have nothing to do with Sirius. Chew the Cud: If you figuratively chew the cud, you're chatting with an acquaintance. If you literally chew the cud, you're regurgitating food from your stomach to be chewed a second time (don't even try it). Cows are ruminants - this means that to properly digest grass to pass through their four-chambered stomachs, they need to rechew it. Consequently, a cow's mouth seems to go nonstop, just like a person who is "chewing the cud." Don't Look a Gift Horse in the Mouth: In other words, don't be ungrateful when someone gives you something. You can tell a horse's age by looking at its teeth, particularly the incisors, but if someone gave you a horse as a gift, it would be considered rude to examine its teeth. (This would be like looking for the price tag on the present.) The Bee's Knees: It's 1920s slang for something wonderful - but why would the knees of the Apis mellifera, the common honeybee, be something to be excited bout? Well, when bees find pollen they carry it back to the hive on pollen baskets located on their hind legs near their knees (yes, bees have knees.) The pollen is then used to make honey. Cold Turkey: To completely abandon an addictive habit is to go cold turkey. As a result, the habit-kicker may experience cold sweats and goose bumps as blood rushes from the surface of the skin to internal organs. That bristling gooseflesh looks like the skin of a plucked goose (which looks quite similar to a plucked turkey). And doesn't it sound better to go cold turkey than to go cold goose?