Ivory is the hard, creamy-white material that forms the tusks of elephants, walruses, and a few other animals. For thousands of years, people carved ivory into jewelry, sculptures, piano keys, and decorative objects because it's beautiful, smooth, and easier to carve than stone.

Elephant tusks are actually elongated teeth made of dentine, the same material inside human teeth but much larger and harder. A full-grown elephant's tusks can weigh over 100 pounds each. Walruses also have ivory tusks, though theirs point downward and help them pull themselves onto ice.

The problem with ivory is that for centuries, hunters killed millions of elephants just to take their tusks, pushing African elephants toward extinction. By the 1980s, so many elephants had been killed that most countries banned buying and selling ivory. Today, scientists estimate that only about 400,000 African elephants remain in the wild, compared to millions a century ago.

The word ivory also describes a color: a pale, creamy white with a slight yellow tint, like old piano keys or aged paper. You might hear someone describe a wedding dress as ivory rather than pure white.

When people say something is like an ivory tower, they mean a person is isolated from real-world problems, living in a protected, privileged place where they don't have to face everyday difficulties.