class_eval.rb

class JiraTicket
  def hello_there
    "hello there"
  end
end

jt = JiraTicket.new

puts "hello there: #{jt.hello_there}"

JiraTicket.class_eval do
  def bye_bye
    "bye bye!"
  end
end

puts "jt has it? #{jt.respond_to?(:bye_bye)}"

puts "what about jt2? #{JiraTicket.new.respond_to?(:bye_bye)}"

#output:
#hello there: hello there
#jt has it? true
#what about jt2? true
#