add_instance_methods.rb

class Message
  [:hello, :goodbye].each do |method_name|
    define_method method_name do |arg|
      "#{method_name} #{arg}"
    end
  end
end

#irb
Message.instance_methods false #=> [:hello, :goodbye]
Message.new.hello 'emre' #=> "hello emre"
Message.new.goodbye 'emre' #=> "goodbye emre"

add_instance_methods_by_instance_method.rb

class Message
  def create_instance_methods *methods
    methods.each do |method_name|
      self.class.send :define_method, method_name do |arg|
        "#{method_name} #{arg}"
      end
    end
  end
end

#irb
Message.instance_methods false #=> [:create_instance_methods]
Message.new.create_instance_methods 'fork' #=> ["fork"]
Message.instance_methods false #=> [:create_instance_methods, :fork]
Message.new.fork 'emre' #=> "fork emre"

add_instance_methods_by_singleton_method.rb

class Message
  def self.create_instance_methods *methods
    methods.each do |method_name|
      define_method method_name do |arg|
        "#{method_name} #{arg}"
      end
    end
  end
end

#irb
Message.singleton_methods #=> [:create_instance_methods]
Message.create_instance_methods 'follow' #=> ["follow"]
Message.instance_methods false #=> [:follow]
Message.new.follow 'emre' #=> "follow emre"

add_singleton_methods_by_instance_method.rb

class Message
 def create_singleton_methods *methods
  methods.each do |method_name|
    self.class.singleton_class.send :define_method, method_name do |arg|
      "#{method_name} #{arg}"
    end
  end
 end
end

#irb
Message.instance_methods false #=> [:create_singleton_methods]
Message.new.create_singleton_methods 'poke' #=> ["poke"]
Message.singleton_methods false #=> [:poke]
Message.poke 'emre' #=> "poke emre"

add_singleton_methods_by_singleton_method.rb

class Message
  def self.create_singleton_methods *methods
    methods.each do |method_name|
      singleton_class.send :define_method, method_name do |arg|
        "#{method_name} #{arg}"
      end
    end
  end
end

#irb
Message.singleton_methods false #=> [:create_singleton_methods]
Message.create_singleton_methods 'like' #=> ["like"]
Message.singleton_methods false #=> [:create_singleton_methods, :like]
Message.like 'emre' #=> "like emre"

get_singleton_class.rb

#ruby 1.9.2
class Message
end

#irb
Message.singleton_class #=> #<Class:Message>

#ruby 1.8.x
class Message
  #define this method to get singleton class
  def self.singleton_class
    class << self
      self
    end
  end
end

#irb
Message.singleton_class #=> #<Class:Message>

ruby_version.bash

$ ruby -v
ruby 1.9.2p136 (2010-12-25 revision 30365) [i686-linux]