An eggshell is the hard, protective outer covering of an egg. It's made mostly of calcium carbonate, the same mineral found in seashells and chalk. Despite feeling fragile, an eggshell's curved shape makes it surprisingly strong: it can support the weight of a sitting hen without breaking, but it can crack easily when you tap it at just the right angle.

The shell has thousands of tiny pores that let air pass through so a developing chick can breathe, while still keeping bacteria out. If you look closely at an eggshell, you might notice it's not perfectly smooth. Some eggs have speckles, and the color varies depending on the bird: chickens lay white or brown eggs, while robins lay beautiful blue ones.

People also use eggshell to describe something extremely delicate or fragile. When you're walking on eggshells around someone, you're being extra careful with what you say because you're worried about upsetting them. You might walk on eggshells around a friend who's having a bad day, choosing your words carefully to avoid making things worse.

In painting and decorating, an eggshell finish describes a surface that's slightly glossy, like the subtle shine you see on an actual eggshell when light hits it.