For animals:
* Shell: This is the most common term, and it applies to animals like turtles, snails, and crustaceans.
* Exoskeleton: This is the hard outer covering of some invertebrates, like insects and spiders.
* Skin: While not always hard, the skin of some animals can be thick and tough, providing protection. For example, the skin of a rhinoceros is very thick and leathery.
For nuts:
* Shell: This is the most common term for the hard covering of nuts like walnuts, pecans, and almonds.
* Husk: This is the outer layer of some nuts, often fibrous and papery. For example, the husk of a coconut is the hairy brown layer that surrounds the hard shell.
It's important to be specific when describing the hard covering. "Shell" is the most versatile term, but "exoskeleton" or "husk" may be more accurate in certain situations.