There are a lot of good ways to learn how to do things in Ruby.  I 
happen to think the Pickaxe (http://www.pragmaticprogrammer.com/ruby/) 
is a great place to start.  But at a certain point, it isn't enough. 
For me, when it comes to open-source languages, the next step is often 
looking at examples, in particular, the standard library.

When I first started doing this with Ruby, one thing hit me right away. 
  Most of the standard library files use the shortcut:

class << self
...
end

This is one of the many ways of defining class methods in Ruby, but 
while it is extremely common in the standard library, it's one that I 
don't believe is mentioned in the Pickaxe.  In addition to this, there 
were all kinds of other interesting, but odd things lurking in the 
standard library, like instance_eval.

So, what I'm wondering is this.  Do other people use the standard 
library as a place to learn how to do things?  If not, why not?  If so, 
which files do you find most useful, most confusing, most well written, etc?

What I've noticed:

singleton.rb: I am amazed by how small it is, but the magic is deep and 
complex.  Using an instance eval, but using the %{} syntax is really 
tricky.  There are no comments to help a newcomer learn, or a maintainer 
maintain, but once you get it, it is pretty obvious what's happening.

date.rb: At first glance, it appears pretty simple, but then there's the 
"once" method.  It is a triky one too.  Once again, no comments to speak of.

profiler.rb: Ouch.  Scary.  I wouldn't expect a profiler to be easy to 
understand, but this one is really opaque.

net/http.rb: Really well documented.  Has some great examples of using 
classes well, and shows a lot of the language flexibility, but the 
comments are somewhat sparse.

Btw, I'm basing this off Ruby 1.6.8.  I haven't gotten around to 
installing 1.8 on this machine yet because I haven't found RedHat 9 RPMs.

Ben