[jballanc]

Well, a trivial example:

    class Foo
      def bar
        puts "Hello from Foo"
      end
    end
    
    a = Foo.new
    def a.bar
      puts "Hello from a special Foo"
    end

Now, even if you change the definition of "bar" in Foo's class definition, "a" will still have it's own version. This might not seem like a big deal, but as you do more metaprogramming with Ruby, more examples like this pop up.