Why are there different sized raindrops?

Isn't it amazing the kind of observation children makes that adults have no time for?

Google results:

http://ga.water.usgs.gov/edu/raindropsizes.html
http://www.madsci.org/posts/archives/1997-03/858795029.Es.r.html
http://www.newton.dep.anl.gov/askasci/wea00/wea00260.htm
http://hypertextbook.com/facts/2001/IgorVolynets.shtml