Actually the class instance variable thing is visible to subclasses, as service= is not called from a subclass directly.

```ruby
irb(main):001:0> class TestClass
irb(main):002:1>   def self.service
irb(main):003:2>     @service = 21
irb(main):004:2>   end
irb(main):005:1> end
=> nil
irb(main):006:0> class SubClass < TestClass
irb(main):007:1> end
=> nil
irb(main):008:0> SubClass.service
=> 21
```