We all know what a shamrock looks like, but—speaking in scientific terms—just what is it? A clover? A wood sorrel? Some other type of plant? Unfortunately, there's no single answer: The shamrock is the stuff of legend, not science, the Smithsonian reports. Over the centuries, a number of different plants have been called the "true" shamrock. It was identified with the clover in the 16th century (and on Wikipedia today), but there are many types of clover. And in the 19th century, a London botanist insisted that a plant called the wood sorrel was the real shamrock. Near the end of that century, an amateur naturalist decided to investigate, asking people from across Ireland to identify the "real" thing. The most popular responses were the yellow clover and the white clover, though the red clover and black medick were also called shamrocks.
In the 1980s, a scientist tried the experiment again and got comparable results, with the yellow clover taking the top spot. And that's the one most often sold to tourists, the Smithsonian notes. As for the significance of the shamrock, even that isn't quite clear: Legend says that St. Patrick compared its three leaves to the Holy Trinity, but that story first appeared in writing centuries later. Regardless, you can take part in tradition by "drowning the shamrock," the Cleveland Plain Dealer reports: After wearing it on your lapel, place it in the night's final drink on St. Patrick's Day. And if that's a pint of Guinness, you'll be in good company, according to WalletHub: Some 13 million pints of the stuff will be downed in celebration. (A Catholic group, however, last year urged people to avoid it.)