This St. Patrick's Day, we're wearing greeen, eating corned beef and cabbage, and listening to the Riverdance soundtrack on repeat.
There's also plenty of Guinness being poured around the world -- about 3 million pints today alone -- and, in the interest of science, we're looking a little closer at the iconic brew.
In particular, scientists have answered the all-important question, "Why do Guinness bubbles sink and not rise?"
Three Irish scientists at the University of Limerick tackled that query in a 2012 paper, "Why do bubbles in Guinness sink?" [PDF]. They found that the secret to Guinness' descending bubbles lies in the shape of the glass:
"If it narrows downwards (as the traditional stout glass, the pint, does), the flow is directed downwards near the wall and upwards in the interior and sinking bubbles will be observed. If the container widens downwards, the flow is opposite to that described above and only rising bubbles will be seen."
It's counterintuitive. As LiveScience put it, "Since bubbles are lighter than beer, one might think this defies the laws of gravity."
It's not a complicated idea, researcher Eugene Benilov told the Ottawa Citizen:
"The bubbles try to rise vertically up. As they do so, they move away from the upward and outward sloping wall, resulting in smaller concentration of bubbles near the wall. The low concentration of bubbles makes the near-wall layer heavier than the rest of the liquid/bubble mixture, so this layer begins to slide down along the wall. Its velocity turns out to be greater than that of the rising bubbles, so they get entrained in the downward motion (even though they still rise relative to the liquid)."
Word to the wise: "Don't drink too much Guinness while testing our conclusions!" Benilov joked with LiveScience. If you do want to test this at home or your local pub, BBC News recommends "a pint of stout... served in a straight-sided, cylindrical glass (not quite filled up)."
(And yes, we're aware that Guinness may not be as Irish as it appears.)
So as you're raising a glass today, say "Sláinte!" to the incredible (and delicious) physics of your pint.
Don't try THIS at your St. Patrick's Day party:
Stay connected with Science Channel on Twitter and Facebook