Click on the photo above to see a video of Brownian motion--the random jiggling of small particles due to molecular bombardment--in this case, the particles are fat globules in diluted milk. You can also see the 3 MB video. Visit Practical Physics to view the same phenomenon in smoke. To find out how a physicist stopped Brownian motion in its tracks, see Hold Still, from the American Physical Society's Physical Review Focus.
image and video credit: David Walker, Micscape; image source