require 'pry'

class Person end

# this goes on an instance of a class
Person.class_eval do
  def say
    puts 'class eval'
  end
end

# this goes on the class is self
Person.instance_eval do
  def say
    puts 'instance eval'
  end
end

#Person.say => instance eval
#Person.new.say => class eval
Pry.start