Birds don't have teeth, paw, hands, antlers, horns, or spines, but they do have beaks.
The beak – also known as the bill – has two parts: the upper mandible and the lower mandible.
The upper mandible is a fancy name for a bird's upper beak or upper bill. The upper mandible grows out of the skull, just as your upper teeth grow out of your skull. The upper mandible doesn't move independently from the skull.
The lower mandible is the scientific name for a bird's lower beak or bill. The lower mandible can move independently because it's hinged. It can move up and down, just like your jaw.
Unlike your teeth, beaks are covered with skin. This skin produces a substance called keratin – the same material feathers, hair, and fingernails are made of. The keratin produced by a bird's beak will dry and condense to make the bill hard and durable. The dried keratin also gives the beak a glossy appearance. As the keratin wears down, it's replaced so the beak will remain sharp.