A chestnut is a smooth, reddish-brown nut that grows inside a spiky green shell on chestnut trees. In autumn, these prickly cases fall to the ground and split open to reveal the shiny nuts inside. People have eaten roasted chestnuts for thousands of years, and they're still popular in many countries. In cities during winter, you might smell chestnuts roasting over street vendor carts, their sweet aroma mixing with the cold air.

The word also describes a reddish-brown color, like the color of the nut itself. A horse with a chestnut coat has this warm, brownish-red coloring.

When someone calls a joke or story an “old chestnut,” they mean it's been told so many times that everyone's heard it before. Picture a joke your grandfather tells at every family gathering: that's an old chestnut.

American chestnut trees once covered the eastern forests, until a disease wiped out billions of them in the early 1900s. Scientists today are working to bring them back, combining the American chestnut's strengths with disease resistance from Asian varieties.