The Science of a Perfect Black and Tan

A Black and Tan — a popular beverage choice on St. Patrick's Day — consists 1 part stout or porter floating atop an equal volume of pale ale. Guinness, an Irish stout, and Bass Pale Ale, an English brew, are most often used, though many variations on this combo exist.

To pour the perfect Black and Tan this holiday, fill a pint glass halfway with Bass. Let that settle, then place an upside-down spoon over the glass and pour the Guinness on top of the spoon's dome. Gently cascading the stout over the spoon and into the ale ensures that it will float on top, rather than splash in and mix with it. True aficionados may wish to purchase a specially designed black-and-tan spoon, which  in the middle so that it can balance on the edge of the pint-glass for easier pouring. [Video: How to Pour the Perfect Black and Tan ]

Ideally, the drink should have between half and one inch of head. Tilting the glass and gradually bringing it vertical as your pour in the Guinness causes this creamy head to develop.

So, what is the physics of the Black and Tan? Just as wood floats on water, stout beer floats on ale because it's less dense. In both cases, you might think the darker substances — wood and stout — would be thicker than their more transparent companions. Not so.

Because the Irish beer sits atop the English one, people often mistakenly believe that the Black and Tan was invented by the Irish as a gag about Ireland's superiority over England. But in fact, this drink is not commonly consumed in Ireland; it has image problems there becaouse of its association with the Royal Irish Constabulary Reserve Force, which operated in Ireland in the early 1920s. Nicknamed the Black and Tans because of the black and khaki uniforms worn by its members, this military unit was charged with suppressing revolution in Ireland and is notorious for its violent attacks on Irish civilians.

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