Hi Marin, you should put the Range ahead the idx var:
(0..8) === idx
That because of the === operator, changes meaning with the object's class which is calling it.
Range.===(Fixnum) works because in the Range class this operator has been implemented to support a comparison with a Fixnum object.
Fixnum.===(Range) doesn't work because in the Fixnum class, nothing has been implemented to make this operator works with a Range object
I would suggest to use an irb shell, to learn about this and other methods and operators