QUESTION:
Since helium rises on Earth, if it rained helium on Saturn, why wouldn't it
rise there, too?

ANSWER from Chris Burrows on April 29, 1996:
Something rises if it is less dense than the medium it is in.  So gaseous 
Helium rises in the earth's atmosphere because it is less dense than the 
air, and rain falls because it is more dense than the air. On Saturn, at 
certain heights in the atmosphere, the temperature is cold enough for 
liquid helium.  Liquid helium is much denser than gaseous helium, so it 
can fall.