> class Foo
    def initialize
      @foo = "foo"
      end
    def foo
      @foo
      end
    end
# => nil
> foo = Foo.new
# => #<Foo:0x22e20dc @foo="foo">
> foo.foo
# => "foo"
> foo.foo = "bar"
NoMethodError: undefined method `foo=' for #<Foo:0x22e20dc @foo="foo">
	from (irb):33
> class Bar
    def initialize 
      @bar = "bar"
      end
    def bar=(value)
      @bar = value
      end
    end
# => nil
> bar = Bar.new
# => #<Bar:0x22ce3ac @bar="bar">
> bar.bar = "foo"
# => "foo"
> bar.bar
NoMethodError: undefined method `bar' for #<Bar:0x22ce3ac @bar="bar">
	from (irb):43
> bar.instance_variable_get("@bar")
# => "foo"