def append(object)
  object << "hello"
end

a = [ 1 ] # array
b = "yo ho ho" # string
c = StringIO.new("yo ho ho") # string IO; still responds to the << method
d = { "hi" => "there" } # hash; does not respond to << method

append a # works
append b # works
append c # works
append d # blows up since there is no << method on hashes