https://twitter.com/Code_60/status/587208979547205632

change.rb

class String
  def to_s
    'String'
  end
end

class Fixnum
  def to_s
    'Fixnum'
  end
end

def greeting(name)
  puts "Hello, #{name}!"
end

greeting('John Doe')
# => Hello, John Doe!
greeting(42)
# => Hello, Fixnum!