What does the generic Animal say?

Beginner programmers on reddit seem to really struggle with the concept of interfaces or pure virtual functions. I thought up a hillarious example to show them what it is all about. I give you, Yelvis.cpp

/* Abstract class Animal and a bunch of concrete implementing classes like Dog, Cat, and Bird goes here */Vector<Animal> animals = getZoo();
for(auto it = animals.begin(); it != animals.end(); it++) {
    it->vocalize()
}/* 
 Prints the following:
    "Dog goes woof
     Cat goes meow
     Bird goes tweet
     Mouse goes squeek
          Cow goes moo
     Frog goes croak
     Elephant goes toot
     Ducks say quack
     Fish go blub
     Seal goes ow, ow, ow" ...but there is one sound that no one knows...
 What does the generic Animal say?
*/