Want to get a beer on a Tuesday? Now you can blame your need to do a scientific experiment. First, though, you need to go to a bar and get a Guinness and some peanuts. And, what the hell, another Guinness.
Now, stare into your beer. Resist the urge to stare at your fellow patrons, or off into the middle distance, and instead stare right at the glass. You'll get a chance to observe fluid dynamics. Look at the bubbles in your pint of Guinness. Are they rising or falling? If you look hard enough, you'll see that they're doing both.
Bubbles, being less dense than the beer that they're suspended in, rise upwards through the beer. In order for that upward movement to be at all noticeable, however, the bubbles need to be moving through beer that isn't falling. Even in a stationary glass, the beer rises and falls.
As the bubbles rise up through it, they create an upward current through the center of the glass. This upward motion can't end in the beer climbing out of the glass, so the liquid needs to fall back down. The liquid that's pushed up through the center of the glass falls back down the sides. The bubbles fall with it. Enough of a good look, and you should see that.
The phenomenon also works in shaken-up particulate solids, which is why, if you shake a bunch of granola, the chunky bits rise to the top. It's particularly noticeable with Brazil nuts. In the case of the nuts, they're too big to fall back down the sides of the package and are stranded on top, which is why you cereal gets progressively boring as you work your way to the bottom of the package. You don't have to worry about that with beer, though. The bubbles come up, and they go down. People recommend Guinness for this experiment because it's dark enough to see the bubbles clearly, but if you want to try it with another brand, go ahead.