Helicopters can fly straight up. They can fly forward, sideways, and backward. They can even hover in one place. An airplane must speed down a long runway to take off and land. Wings hold an airplane in the air. Helicopters do not need runways, and they do not have wings.
WHAT HOLDS A HELICOPTER IN THE AIR?
Big blades on top of a helicopter keep it in the air. The blades are a little like fan blades. The blades spin very fast. Wind blowing down from the whirling blades holds a helicopter up. The blades also control the direction in which the helicopter flies.
Do you play computer games over the Internet? Do you surf the Web? Do you send e-mail messages to your friends? You can get all kinds of information on the Internet. People use the Internet to work at home. Scientists use the Internet to help them do research. The Internet has made big changes in the way many people live and work.
WHAT IS THE INTERNET?
The Internet is a system that connects computer networks. The Internet links millions of computers all over the world.
How is it possible for something right in front of your eyes to be completely invisible? It’s possible when that thing is too small to be seen with your eyes.
We can see drops of water from a pond or lake. But we cannot see the thousands of tiny creatures that live in the water. Drops of blood contain tiny structures called cells, but we can’t see them either. Microscopes allow us to see invisibly small things.
Microscopes are one of the most important tools of scientists. Medical scientists use them to see the germs that make people sick. Biologists use them to see how plants and animals are constructed.
Try to imagine what life would be like if there were no electric lights, no television, or no cars. What if you had no running water to take a shower and no refrigerator to keep your food cold? Life would be tough without these great inventions.
Since ancient times, people have been inventing new machines, materials, and ways of doing things. The people who come up with new things are called inventors. Sometimes an inventor makes an invention to solve a problem. Sometimes an inventor discovers something new by accident.
First you see a blinding flash of light, brighter than the Sun. Moments later, a huge ball of fire appears, brilliant orange. The fireball begins to rise into the sky. Soon it widens at the top and is shaped like a mushroom. A thundering sound and blast of heat reach you 15 miles (24 kilometers) away. You are seeing the explosion of the world’s first nuclear weapon, on July 16, 1945, in a New Mexico desert.
WHY DO WE HAVE NUCLEAR WEAPONS?
Nuclear weapons are the most destructive weapons ever made. Building a nuclear weapon was a top-secret project during World War II.