ITP2 - OOP Tutorials - Classes and Objects II - Exercises

When running the following code...

class Person
  attr_reader :name
  def initialize(name)
    @name = name
  end
end

bob = Person.new("Steve")
bob.name = "Bob"

We get the following error...

test.rb:9:in `<main>': undefined method `name=' for
  #<Person:0x007fef41838a28 @name="Steve"> (NoMethodError)

Why do we get this error and how to we fix it?