Years ago, dinosaurs roamed the Earth. But while there are no dinosaurs alive today, there are creatures that are related to dinosaurs — reptiles such as snakes and alligators, as well as some fish, plants and other creatures.

We know that dinosaurs died out millions of years ago, but where did today’s reptiles come from?

The world was very different when the dinosaurs were alive — it was much hotter and wetter. Over time, the world changed and it became harder for dinosaurs to survive. Some scientists believe that an asteroid (a large rock from space) may have hit the earth in the time of the dinosaurs, causing the temperature to drop suddenly.

When big changes like this happen, some creatures are well suited to survive, and others aren’t. Creatures that are not well suited to survive in the world will die out. A creature that is well suited to survive will have lots of babies that will often be even better suited to survive.

After the earth cooled during the time of the dinosaurs, most dinosaurs died out. Those few who survived had babies who were smaller, smarter, and quicker. As time went on, those babies had babies of their own, and over the course of thousands of years, these creatures didn’t look like dinosaurs at all anymore. They had changed into new kinds of animals, and these animals were well suited to survive in the changed world.

All types of creatures change slowly over time. Scientists call this process evolution.

Here is a video from NASA, the American government agency that is responsible for the space shuttle and other rocket launches. The video shows the space shuttle’s solid boosters returning to earth. The solid boosters are two of the space shuttle’s gas tanks. When the space shuttle is high enough off the earth, the boosters break off from the space shuttle and land in the ocean. This video takes about five minutes to watch. It follows the boosters from the time the break away from the space shuttle to the time they splash down in the ocean. Be sure to watch all the way to the end to see a surprise.

A volcano is a special kind of mountain. Volcanos are special because they can erupt. When a volcano erupts, it can sent hot lava, steam, ash, and rocks into the air.

The heat from a volcano comes from deep within the earth. Many miles underground, the earth is hot. Rocks beneath the earth are so hot they turn into a liquid called lava. When a volcano erupts, the steam and ash is caused by lava that is forcing its way toward the surface of the earth.

We don’t find volcanos in every part of the world. In the United States, most of the volcanos are in Hawaii and Alaska. There are quite a few volcanos in Asian countries such as Japan. The largest active volcano in Europe is called Mount Etna, in Italy.

You don’t have to be worried about being hurt by a volcano. Volcanos erupt very rarely, and most volcanos don’t hurt anybody when they erupt.

Doctors know a lot about how to help people who are sick or injured. But how do they learn how to help people? It takes years of special schooling to learn how to be a doctor. In medical school, doctors learn about peoples’ health from books and classroom discussions, but they also learn by doing experiments, just like other scientists.

To really learn about what’s inside our bodies, student doctors sometimes cut up dead bodies. Bodies that are used by student doctors are called cadavers. Before they die, people sometimes give permission to medical schools to use their bodies to teach doctors. Cutting up a dead body may seem gross, but doctors become accustomed to it. Using cadavers is the best way for a doctor to learn what goes on inside our bodies.

You’ve probably noticed that sometimes when you cut yourself, blood comes out. But what is blood, and why does it come out when we cut ourselves?

Blood is made of many different parts. These parts of the blood are tiny — smaller than you can see. One part of blood is made of red blood cells which carry the air you breathe throughout your body. Red blood cells also give blood its red color.

You can see what blood cells look like under a microscope in the picture at right. (Because this picture was taken by a microscope, which makes tiny things look much larger, the cells look bigger than they do in real life.)

When you get a cut, sometimes tiny creatures that are too small to see can try to get inside your body through the cut. If some of these creatures get into your body, they can cause an infection, which makes the cut hurt a lot more. Your blood helps keep those bad creatures out. There’s another part of your blood, called white blood cells, which help keep the bad guys out of your body. Like red blood cells, white blood cells are also too tiny to see. The job of a white blood cell is to prevent bad creatures from coming into your body through a cut and infecting you.

Whenever you cut yourself, you should get help from an adult (like one of your parents or a teacher) to clean the cut and put a bandage on it.

Getting a cut is no fun, and you probably don’t like to see blood come out when you cut yourself, but remember that the blood is there for a good reason.

Have you ever seen a shooting star? Sometimes late at night you can see a streak of light blaze across the sky, as in the picture at right.

Is a shooting star really a star? Not really. Real stars (including our sun) are giant balls of fiery gas. Shooting stars are actually much smaller than real stars, and they’re made of rock or metal. They look like stars because they fly through the air so quickly that they become hot and glow. You can see how this works by rubbing your hands together. The same force that makes a meteor hot as it flies through the air is what makes your hands hot when you rub them together. This is called friction.

Scientists have a special name for shooting stars: they call them meteors.

You don’t have to worry about being hit by a meteor. Most meteors burn up before they hit the earth.

You’ve probably noticed that it gets warmer in summer and colder in the winter. This change happens every year. It happens because the earth tilts back and forth as it goes around the sun. During the summer, the earth tilts toward the sun, which makes half of the earth hotter — this is what we call summer. During the other half of the year, the earth tilts away from the sun, which makes that half of the earth cooler. We call this winter.

Did you know that different parts of the world have winter and summer at different times of the year? In the northern half of the world (such as North America and Europe), winter happens during the months of December, January and February. In the southern half of the world (such as South America and Australia), winter happens during the months of June, July and August. Winter happens at different times because the top and bottom halves of the earth tilt away from the sun at different times of the year.