Duck typing, explained with Ducks.

duck-typing.rb

# This is a Duck. It is clearly not a string.
class Duck
  def to_s
      "   _  \n" +
      "__(.)<\n" +
      "\\___) \n"
  end
end

# This displays Strings. It will clearly never display a Duck.
def print_if_string(x)
  unless x.is_a?(String)
    puts "A #{x.class} is not a String."
    return
  end

  puts x
end

print_if_string(1) #=> "A Fixnum is not a String."
print_if_string("This is definitely a String") #=> "This is definitely a String."

print_if_string(Duck.new) #=> "A Duck is not a String"

class Duck
  def is_a?(other)
    super(other) || other == String
  end
end

print_if_string(Duck.new) #=> A duck.

output.txt

~$ ruby ./duck-typing.rb
A Fixnum is not a String.
This is definitely a String
A Duck is not a String.
   _  
__(.)<
\___) 
~$