Examples of 'Things' that are not Objects in Ruby

Question

"Everything is an object" was one of the first things I learned about Ruby, but in Peter Cooper's Beginning Ruby: From Novice to Professional, it is mentioned that "almost everything in Ruby is an object".

Can you give me some examples of things that are not objects in Ruby?

Answer 1

The most obvious one that jumps into my head would be blocks. Blocks can be easily reified to a Proc object, either by using the &block parameter form in a parameter list or by using lambda, proc, Proc.new or (in Ruby 1.9) the "stabby lambda" syntax. But on its own, they aren't objects.

Another example are operators.