An obelisk is a tall, four-sided stone pillar that tapers as it rises and ends in a pyramid-shaped point at the top. The ancient Egyptians invented obelisks over 4,000 years ago, carving them from single massive pieces of granite. They covered these monuments with hieroglyphics celebrating pharaohs and gods, then placed them at temple entrances where they caught the sun's rays.

The most famous obelisk-shaped monument in America stands on the National Mall in Washington, D.C.: the Washington Monument, built to honor George Washington. At 555 feet tall, it's one of the world's tallest stone structures and the tallest obelisk-shaped monument ever built. You can spot its distinctive shape from miles away.

Many ancient Egyptian obelisks now stand in cities around the world. One called Cleopatra's Needle stands in Central Park in New York City, while another stands on the Thames Embankment in London. Rome has more ancient Egyptian obelisks than Egypt itself, brought there by conquering Romans 2,000 years ago.

When you see that tall, tapering shape ending in a pyramid point, whether ancient or modern, you're looking at an obelisk.