If it looks like a duck, swims like a duck and quacks like a duck, it could be exquackly the kind of dog or duck that fits the bill for a dog`s best friend.
After all if it looks like a duck, swims like a duck and quacks like a duck, you don't need to canvas back and forth wandering and whistling in hopes of spotting an uncommon duck to fit the " bill" .
So a duck is an animal, you know, that says: " Quack quack" , at least in English it does, and a lot of students, when they say these words they pronounce them the same way.