I was trying to write a simple array flatten method, but it does not work using instance variable. It works only using class variables. Can anyone tell me why? and how to make it work using instance variables.

 class Array
 @y = []
  def flatten_array
   self.each do |x|
    if x.class.to_s != 'Array'
    @y <<  x
     else
     x.flatten_array
    end
   end 
   return @y  
  end  
 end


 a =  [1,2,3,4,5]
 b =  [6,7,8]
 c =  [9,10]
 a1 = [12,13,a,b,c]
 puts a1.inspect
 b1 = a1.flatten_array
 puts b1.inspect