Could someone help me with the following structure:
 
as+adj/adv+as
so+adj/adv+as
We can say "there is nothing as/so strong as an idea whose time has come"
 
And we can say. "Bob is as tall as Fred."
 
Both appear to be comparisons, but you cannot say "Bob is so tall as Fred."
I know there's a difference here but I can't put it into words.
Help!