To molt means to shed old feathers, fur, skin, or shell and grow new ones to replace them. Birds molt their feathers, snakes molt their skin, and crabs molt their hard shells. When your dog leaves hair all over the furniture in spring, that's a type of molting too.

Molting happens because animals outgrow their outer coverings or need fresh ones. A snake can't stretch its skin as it grows, so it regularly molts the old skin in one piece, leaving behind what looks like a ghostly snake costume. Birds molt damaged feathers so they can fly properly, and some birds molt into completely different colored plumage for different seasons. Cardinals stay red year-round, but goldfinches molt from bright yellow in summer to dull olive in winter.

The process can take weeks or months. During a molt, animals are often vulnerable: a bird with missing feathers might not fly as well, and a crab with a soft new shell needs to hide from predators until its shell hardens.