Of all the planets in our solar system, Earth is the only one that can support human life. No other planet has oceans of water, an oxygen-rich atmosphere, and intelligent life.

Earth is special in many less dramatic respects, too. It is the largest body in the solar system with a solid surface and the only planet with ongoing plate tectonics -- the slow but constant shifting of large, fairly rigid plates on the planet's surface. These two facts are linked, since Earth's solid surface locks in the heat necessary to force the crust to move.

In other ways, however, Earth is just an average planet. It is bigger than four planets in the solar system and smaller than the other four. Two planets, Venus and Mercury, are closer to the Sun, while the other six are farther away.

Earth formed about 4.5 billion years ago, and it is still evolving. Earthquakes and volcanoes are violent proof of Earth's shifting crust. Beneath this thin crust of rock -- which averages about four miles (7 km) thick under the oceans and up to 25 miles (40 km) beneath the continents -- lies a 1,700-mile (2,700-km) layer of denser rock called the mantle.

Beneath the mantle is Earth's core, made mostly of nickel and iron. The inner part of the core is solid, but the outer part is liquid. From its edge to its center, the core measures about 2,000 miles (3,220 km).

Oceans cover more than two-thirds of Earth's surface. The first signs of life on Earth appeared in the oceans about 3.5 billion to 3.9 billion years ago. The first tiny organisms were similiar to forms of bacteria still living today. The first life on land, simple plants, developed around 400 million years ago. The age of the dinosaurs followed 200 million years later, and human beings evolved less than one million years ago.

Above Earth's solid surface lies its atmosphere. The atmosphere has more oxygen today than it did when Earth formed. While the early atmosphere was mostly carbon dioxide, more than three-quarters of Earth's atmosphere is now nitrogen, and most of the rest of it is oxygen. Chemical reactions locked much of the early carbon dioxide inside rocks, while plants produced the oxygen in our present-day atmosphere.

Human technology is another factor that sets Earth apart from other planets. Thousands of artificial satellites orbit our planet, enabling humans to communicate with one another, monitor the weather, and, sometimes, keep an eye on other people.

Gravity holds you "down" on the planet's surface and makes you feel heavy. The oxygen in the atmosphere allows you to breathe comfortably, and the Sun keeps you warm in the daytime.

A huge variety of living things populates Earth -- plants, birds, fish, mammals -- while not even one living organism has been found on any other planet or moon in our solar sytem.

By watching the sky, you can see changes in Earth's atmosphere, as clouds form and move, rain falls, and lightning strikes. Water droplets in the atmosphere produce rainbows when sunlight passes through them at the proper angle. People in the far north and south can see fantastic displays of light called aurorae, caused by energetic particles from the Sun trapped in Earth's magnetic field.

In Earth's night sky, you can look out through the atmosphere and see stars and galaxies many trillions of miles away. You can see the Moon, our own companion world. And you can look at all the other planets in our solar system -- interesting places to visit, but not as nice to live on as Earth.