Pour a stout and watch the side of the glass while it settles: the bubbles drift down. Bubbles can’t sink, and these don’t. In 2012 three mathematicians at the University of Limerick, Eugene Benilov, Cathal Cummins and William Lee, worked out what is going on. A pint glass is wider at the top, so its wall leans outward. Every bubble rises straight up, which means the ones near the wall move away from it, leaving a thin layer of beer by the glass with no bubbles in it. Beer without bubbles is heavier than beer with them, so that layer slides down the wall, and the bubbles beside it are so small that they get carried along. Up the middle, the rest of the pint rises to make room. Put the same stout in a glass that narrows towards the top and the whole thing runs the other way: the bubbles by the wall go up, like any sensible bubble. What you were watching was never the bubbles. It was the beer, and the shape of the glass told it which way to go.