Energy is the ability to do work. Solar energy comes from the Sun. There is also energy that comes from wind and water.

But do you know that energy can come from people, too? When you pedal a bike, you produce energy. You use your legs to push the pedals. Your energy is transferred (转移) to the bike. That makes the bike move.

Now imagine riding your bike to create enough power to run a computer. Some students at one school did just that! They jumped on bikes connected to generators (发电机) and pedaled in place. Soon they were producing electricity. This energy is needed to run their computers.

Here is how bicycle-powered energy works. When a student pedals the bike, the back wheel spins. The wheel spins the generator. The generator produces electricity. As long as a student pedals the bicycle, electricity moves through the generator. The electricity can be used right away. This energy can also be stored and used later.

People use bicycle-powered energy to run small devices (设备) such as small televisions and fans. They often use less than 100 watts per hour. However, it would be a mistake to use pedal power to run a fridge. This large machine often uses more than 700 watts per hour. The electricity for these machines comes from power lines overhead or underground.

Pedal power can be seen in schools, gyms, and homes. What a fun way to provide electricity!

2 . According to Ken Croswell, the Sun and the Moon are different. We often think that the Sun rules the day and the Moon rules the night. That’s kind of right, but not totally.

The Sun shines all the time. It shines during the day when we see it, and even at night when we don’t.

As the Earth turns, sometimes we face the Sun. That’s when we get day. When the Earth’s spin (自转) turns us away from the Sun and we are in the Earth’s shadow, we get night. When people on the opposite side of the Earth face the Sun, those people have day while we have night.

The Moon is a different story. Unlike the Sun, it doesn’t make its own light. Moonlight is actually the Sun’s light reflecting(反射) the Moon back to us.

The Sun lights up only the side of the Moon that faces the Sun. At Full Moon, the Moon is on the opposite side of the Earth from the Sun. Then the side of the Moon facing the Earth is all lit up. During Full Moon, the Moon rises around sunset, shines all night, and sets around sunrise. Since he moonlight is so bright, you can sometimes see it during the day, too. A few days before Full Moon, if you look east in the afternoon, you may see the Moon in the sky. After Full Moon, you may find it if you look west in the morning.

The Moon is most difficult to see at New Moon, when its orbit(轨道) puts it between the Earth and the Sun. Then, the side of the Moon facing the Earth is dark and we can’t see the Moon at all.

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3 . There’s nothing quite like falling sound asleep after a full day of work. Like people, animals need to rest after working hard. Some animals sleep in water. Others dig holes under the ground. Some even sleep high in trees or under leaves. But they all find a way to rest.

Some animals in the sea sleep in strange ways. Fish sleep with their eyes open. They seem to look ahead far into the sea while they rest. Sea otters sometimes sleep in beds of the plants that grow in the sea. This keeps them from moving away. Parrotfish blow something from their mouths when they are ready to sleep. Then a bubble (气泡) will be formed around them. The bubble protects them from harm while they sleep.

Some animals sleep under the ground. Chipmunks (花栗鼠) sleep curled up (蜷缩) in a ball. Their beds are made of leaves and grass. In winter, they sleep for two weeks and wake up to eat the stored food. Then they go back to sleep for another two weeks before waking up again. Some desert frogs dig underground holes during the hot, dry season. A frog may stay in its hole for months.

High above the ground, monkeys find leafy places in trees each evening before they sleep. The tall trees help to keep monkeys safe during the night. Some insects even sleep under the leaf that will be their next meal. A bee may climb down into a flower to rest. When it climbs out the next morning, it is rested and ready for work.

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4 . Every living thing needs to reproduce. Reproducing means creating more members of your group. In order for plants to reproduce, they have to spread their seeds (种子) to other areas. Plants have developed all kinds of ways to do this.

The simplest way is gravity. Many seeds are inside of fruit. When a piece of fruit gets too heavy, it falls from a tree to the ground. Sometimes animals will pick up the fruit and drop it in another area. That helps move the seed even farther away. Apples spread this way.

Other plants use wind to spread their seeds. Dandelions are a good example of this. Dandelion seeds are so light that when wind blows, it carries dandelion seeds to new places. Maple (枫树) seeds also use wind. Their seeds are connected with long, thin leaves that look like wings. When the seed falls from the tree, its “wings” help it fly farther from the tree.

Some animals help plants spread their seeds. The animals eat the seeds. While the seed is in the animal’s body, it stays whole. When the seed leaves the animal, it’s in a new place.

Ants also help spread seeds. Some seeds have a special smell that attracts ants. The ants bring the seeds back to their home, which of course is underground. They only eat part of the seed. They leave the rest of the seed underground. After that, the seed can start growing.