A horseshoe is a U-shaped piece of metal that protects a horse's hoof. Horses in the wild naturally wear down their hooves as they walk, but horses that work hard, carry riders, or travel on roads need extra protection. A specialized craftsperson called a farrier shapes the horseshoe from iron or steel, heats it until it's red-hot, then nails it onto the horse's hoof. This doesn't hurt the horse any more than cutting your fingernails hurts you, since hooves are made of the same material as your nails.

People have nailed horseshoes to horses' feet for over 2,000 years. Before horseshoes, armies couldn't march long distances and farmers couldn't plow rocky fields without their horses going lame. The invention of the horseshoe helped build empires and feed civilizations.

Horseshoes also appear in games and traditions. In the game of horseshoes, players toss U-shaped metal pieces at a stake in the ground, trying to ring it or land closest. Many people hang horseshoes over doorways for good luck, with the open end pointing up so the luck doesn't “fall out.” Whether this brings actual good fortune is debatable, but the horseshoe's practical importance in human history is beyond question.