Instance vs Class variables and Random Numbers Ruby

Hello I am writing a code for a simple die of (n) sides and it works fine:

class Die

  def initialize(sides)
    @sides = sides
    raise ArgumentError if sides < 1 
  end

  def roll
    roll_number = rand(@sides) + 1
  end
end

However, when I change my initialized variable from instance to class the random numbers generated go from integers to a bunch of small floating point numbers around 1.

class Die

  def initialize(sides)
    HERE --> @@sides = sides
    raise ArgumentError if sides < 1 
  end

  def roll
    roll_number = rand(@sides) + 1
  end

end

I wanted to know what may cause this difference?