Knowledge of the world
Science of craft

Science and technology is a topic that encompasses sciencetechnology, and the interactions between the two.

Science is a systematic enterprise that builds and organizes knowledge in the form of explanations and predictions about nature and the universe.

 Technology is the collection of techniques, methods or processes used in the production of goods or services or the accomplishment of objectives, such as scientific investigation, or any other consumer demands.

Endangered and Extinct Animals

You have never watched a dodo bird scurry along the ground. You have never been frightened by a saber-toothed cat crouched in a tree. You have never seen the huge tusks of a mastodon. These animals once lived on Earth, but they all died out. They went extinct.

You can still see blue Whales, giant Pandas, and tigers. But these animals are in danger of becoming extinct. There aren’t many of them left. Blue whales, giant pandas, and tigers are some of the endangered animals on Earth. There are thousands of others.


Different kinds of animals have appeared and disappeared throughout Earth’s history. Some animals go extinct because the climate where they live changes. The climate may become wetter or drier. It may become warmer or cooler. If the animals cannot change, adapt, to the new climate, they die.

Some animals go extinct because they cannot compete with other animals for food. Some animals go extinct because they are killed by enemies. New kinds of animals are always evolving. Evolving means that the animals are animals are changing slowly from generation to generation. Small differences between parents, children, and grandchildren slowly add up over many, many generations. Eventually, a different kind of animal evolves.Sometimes many of the animals on Earth go extinct at the same time. Scientists call this a mass extinction. Scientists think there have been at least five mass extinctions in Earth’s history. The last mass extinction happened about 65 million years ago. This mass extinction killed off dinosaurs.


Scientists are not sure what caused the extinction of the dinosaurs. Many think an asteroid or comet may have crashed into Earth. The crash would have made a huge dust cloud. The cloud could have blocked out the Sun’s light for years. Without light, plants, dinosaurs, and other animals would die.

Many new animals, including large mammals evolved after the dinosaurs died off. Scientists think that mass extinctions make way for new kinds of animals to develop.


Scientists study fossils to learn about extinct animals. Fossils can be imprints of the animals left in rock. They can be bones that have turned to stone.

Scientists study fossils to learn what the animals looked like. They study other rocks around the fossils to learn when the animals lived. Some fossils are millions of years old.


Many animals are currently in danger of going extinct. Human beings are causing thousands of kinds of animals to die off. Chemicals that pollute the environment can endanger animals. Hunter kill some animals. Hunter kill rhinoceroses for their horns. They kill tigers for their skins. They kill elephants for their ivory tusks.

Most endangered animals today are in trouble because people are destroying their habitats. A habitat is a place where an animal normally lives. Forests, deserts, and wetlands are kinds of habitats. Many endangered animals live in tropical rainforests. People cut down rainforests to build farms, roads, and towns. They drain water from wetlands. Gorillas and koala bears are endangered because of destruction of the forests in which they live.


Governments can pass laws that make it illegal to hunt or sell endangered animals. They can pass laws against using chemicals that harm certain animals. DDT was a bug killer that once endangered the bald eagle and the peregrine falcon. DDT was banned, and the populations of these birds increased.

Sometimes people try to save one kind of endangered animal. They take some of the animals to zoos or laboratories. They breed the animals there. When there are enough animals, they set them free in the wild. Biologists are trying to save giant pandas and the California condor in this way.

Some biologists believe the best the best way to save endangered animals is to save their habitats. They work set up wilderness areas such as parks and national forests where the animals can live. They keep the animals safe from hunters and other dangers.


Asteroids are rocks in space that never quite made it as planets. Astronomers think that our solar system began as a cloud of gas and dust. Gravity pulled parts of the cloud together to make the Sun and the nine planets. Astronomers think that the asteroids formed in that cloud but never grew large enough to be planets.


There are thousands of asteroids, and they come in all sizes. The biggest asteroid ever found is called Ceres. Ceres is more than 600 miles (1,000 kilometers) wide. Astronomers have found about 200 asteroids that are more than 60 miles (100 kilometers) across. All the other asteroids are much smaller. Some are only a few feet wide.

Astronomers wonder if once there were just a few big asteroids. The big asteroids may have crashed into each other. The crashes would have broken them into smaller pieces, making all the asteroids we see today.

Some asteroids are round. Some asteroids are long and bumpy. Some asteroids even have tiny moons going around them.


Asteroids go around, or orbit, the Sun just like planets. Most asteroids orbit in the asteroid belt. The asteroid belt is farther out from the Sun than Earth’s orbit. It lies between the orbits of the planets Mars and Jupiter.

Sometimes asteroids change orbits and move out of the asteroid belt. These asteroids cross the orbits of planets as the planets go around the Sun. A few cross Earth’s orbit.


Asteroids are normally too small and far away to see with your eyes. Astronomers study asteroids with telescopes. They have also sent spacecraft for close-up looks at several asteroids. A spacecraft named Near-Earth Asteroid Rendezvous (NEAR) Shoemaker actually landed on an asteroid in 2001.

Astronomers have found that most asteroids are made mainly of stone. Some asteroids are made mostly of metals. Astronomers think that asteroids made of metal must have melted at some time in the past. The liquid metal clumped together at the center of the asteroid. Most of the rocky part later broke off from the asteroid, leaving the metal behind. What melted these metal asteroids is still a mystery.


Astronomers think that several thousand asteroids have orbits that might one day make them strike Earth. Asteroids have certainly hit Earth in the past. People have found thousands of meteorites (stones from space) that have crashed into Earth. Most meteorites are pieces of asteroids. There is a giant crater (hole in the ground) in Arizona that is more than half a mile (about a kilometer) wide! The crater was formed when a meteor crashed into Earth.

An asteroid crashing into Earth may have killed off the dinosaurs 65 million years ago. Scientists have found a big meteorite crater around Mexico’s Yucatán Peninsula. They think the asteroid that created this crater may have killed the dinosaurs. After the asteroid crashed, a huge cloud of dust would have darkened Earth. It could have become very cold on Earth, and plants that dinosaurs ate might have died. As the plant-eating dinosaurs died from lack of food, meat-eating dinosaurs would have run out of food and died as well.

Scientists are setting up a system to warn us of asteroids coming toward Earth. If they find one they might be able to blow up the asteroid. They might be able to attach a rocket to the asteroid and push it just enough to miss Earth.

Black Holes

Black holes are some of the strangest things in space. A black hole sucks in anything that gets near it. Nothing can escape from a black hole not even light.


Nothing escapes from a black hole because its gravity is so strong. Gravity is a force that pulls one thing to another. Gravity is the force that holds you down on Earth. When you jump up, Earth’s gravity pulls you right back down. Earth’s gravity also makes the Moon orbit (go around) Earth.

The more matter (stuff) that is packed in a star, planet, moon, or other object, the stronger is its gravity. Gravity makes an object with more matter pull an object with less matter toward it. The Sun has a lot more matter than Earth. The Sun’s gravity pulls on Earth. It makes Earth orbit the Sun.

Matter is packed very tightly in some things and loosely in others. The matter that makes up an iron ball is packed much tighter than the matter that makes up a bag of feathers. A scientist would say that an iron ball is much denser than a bag of feathers.

A black hole is denser than anything you could imagine. A black hole could have a million times more stuff than our Sun. All of this stuff would be packed into an area smaller than a city. The force of gravity from so much stuff packed into such a small area is awesome.


Astronomers and physicists think black holes come from dying stars. A dying star burns out and stops shining. All the stuff that makes up the star starts falling in on itself. The star gets denser and denser. If the star is big enough and has enough matter, it could get dense enough to become a black hole.


No one has really seen a black hole. You cannot see black holes because they do not give off any kind of light. Physicists used math to predict that black holes exist.

Astronomers look for signs of black holes. Astronomers study powerful rays coming from stars in deep space. The stars seem to be orbiting black holes. Astronomers think that black holes are sucking gas from the stars, and this makes the stars give off X rays.

Galaxies are enormous groups of stars. Astronomers think that most galaxies have huge black holes at their centers. The Hubble Space Telescope took pictures of a disk of hot gases at the center of our own Milky Way Galaxy. Astronomers think this disk is going around an enormous black hole right in the center of our galaxy.