Hundreds of years ago, a towering temple stood at the center of what is now Mexico City. This Great Temple was a symbol of the power of the Aztec Empire—an empire that stretched across much of Mexico.

Would you put on your bathing suit and jump into icy water? Would you slide down a snowy hill on your bare stomach? Imagine how cold that would feel! You would freeze. Penguins, however, do this all the time. Penguins swim in cold water. They slide on their bellies over ice and snow.

Riding a bicycle is a great way to get around. Bicycles are quiet, fun to ride, and you don’t need gas or electricity to make them go. You just use the power of your legs!
Bicycles have two wheels. One wheel is in front of the other. You swing your leg over the bike, sit down, and start pedaling.

The Mississippi River is the most important river in North America. It provided a major highway for early explorers of North America. Many cities grew up along it. Furs and farm goods traveled from these settlements down the river to markets. Today, more freight travels on the Mississippi than on any other waterway within the continent.

Geronimo was a leader of the Apache Indians. He belonged to an Apache group known as the Chiricahua. In the 1870s, the United States government tried to force the Chiricahua off their homelands and onto reservations. Geronimo fought back.

Stand very still. You may think you are not moving, but the ground may be moving ever so slightly under your feet. The ground you are standing on is part of Earth’s crust. The crust is the solid surface or outer, rocky layer of Earth. Pieces of Earth’s crust are always slowly slipping and sliding around. The idea that pieces of Earth’s crust move is called the theory of plate tectonics.

The sound of an orchestra playing is a thrilling experience. An orchestra is a group of musicians who play musical instruments. Some orchestras have more than 100 musicians. Yet when they play together, the orchestra sounds like one mighty musical instrument.

Try to jump as high as you can. Bend your knees. Now jump! No matter how hard you try, or how high you jump, you always come back down again.
Something called gravity pulls you back down. Gravity holds you down on Earth. Without gravity, you would fly off into space. You would jump up and just keep on going. That might sound like fun, but you could not live very high up. For one thing, you need to breathe air. The higher you go, the less air there is. You need gravity to keep you down on Earth.

Remember Mary’s little lamb? It followed her everywhere. Lambs follow their mothers. If a newborn lamb loses its mother, it will form a bond with a human being who adopts it. Just like Mary’s lamb, it will try to tag after its human parent. Lambs are baby sheep.