Our $2 Tuesday scientific experiment:
Now, stare into your beer. 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.
If you need a little help, try putting peanuts in your beer. This is why you'll need at least two pints. The bubbles adhere to the surface of the peanuts and lift them up. Once they get to the top of the beer, the peanuts should be carried towards the side of the glass. On the surface of the beer, the bubbles burst, and the peanuts slide back down.