Adding to the Array class' instance methods:

class Array
   def my_method
   end
end

Array.instance_methods.grep /my_meth/ # => [:my_method]

Adding to the Array class itself:

class Array
   # More than one idiom in use here..
   def self.my_other_method
   end
end

Array.methods.grep /my_oth/  # [:my_other_method]

greetings,
kaspar