Those are so cool. They're the only way (I know of) that you can see molecules with your naked eye. Actual molecules! That's practically at the level of atoms, and you can see that microscopic stuff!
**Edit: they are not, in fact, molecules. See replies for more info. **
Also red blood cells. It's pretty cool that if you stare at the sky (not the sun; please not the sun) you can sometimes see red blood cells move in the back of your eye. Single cells! Amazing.