There is little rain in desert. Because deserts are so dry, they have no "quilt" to help stop the soil from going away. As a result, they may get very hot during the day with the sun shining, but don't hold the heat overnight. Many deserts can quickly get cold once the sun goes down. Some deserts can reach temperatures of over 100 degrees F during the day and then drop below freezing (32 degrees F) during the night.

The largest hot and dry desert in the world is the Sahara Desert in Northern Africa. The Sahara is a sandy desert with great sand hills. It covers over 3 million square miles of Africa. Other large deserts include the Arabian Desert in the Middle East, the Gobi Desert in Northern China and Mongolia, and Kalahari Desert in Africa.

Animals that live in the desert are also used to needing little water. Many get all the water they need from the food they eat. Other animals keep water that they can use later. The camel stores up fat in its hump while other animals keep something they need in their tails.

Only certain types of plants can live in the terrible environment of the desert. You won't see a lot of tall trees in the desert. Most plants have a way to keep water in their leaves, or trunks so they can live a long time without water.

Now deserts cover around 20% of the world's land, but they are growing. This is called desertification and is caused by different reasons including human activities. The Sahara Desert is growing larger and larger each year. What should we do with it ?

   Have you ever rubbed your hands together when they get cold? Why do we do that? We do that for friction. Friction happens when two things that aren't very smooth rub together to make heat energy. The faster you rub two things together, the more friction (and heat) you can make. Let's discover friction!

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    Paul is a photographer(摄影师). He went to the Arctic to take pictures for NationalGeographic(《国家地理》). Finally, he saw a big and white polar bear swimming to him in the icy water.

    Paul lay down the ice to get a better photo, but the polar bear jumped out of the water and tried to attack him! Luckily for Paul, the ice cracked(破裂), or broke and the bear fell back into the water. The polar bear wasn't so lucky. It couldn't get a meal and had to swim farther away to find food.

    To understand the polar bear's problem, you need to know two things about the Arctic. First, the Arctic is mostly ocean(海洋). Second, ice covers(覆盖) a lot of the Arctic Ocean. In winter, the ice is thick, but in summer, the ice becomes thinner and breaks into pieces. Sea ice is the key to survival for Arctic animals. However, because of global warming, Arctic temperatures are rising and polar ice is melting(融化).

    Polar bears need ice  to hunt(捕捉) seals, their favourite meal. They stand on the ice and wait for the seals to come up for air. As Arctic ice melts, polar bears have to swim farther to find ice floes, or large pieces of floating ice. Sometimes, bears have to swim more than 200 km to find an ice floe. Some bears cannot swim that far and sadly they drown(淹死).

    On his last trip, Paul saw three dead bodies of polar bears. Before global warming threatened(威胁) Arctic ice, a dead bear was a rare sight. Paul is sad to see the dead bodies. Wildlife experts(专家) think global warming will have a terrible effect(影响) on polar bears. In the future, polar ice will continue to shrink and endanger the bears. Some scientists believe 65 percent of the world's polar bears will disappear(消失) by 2050. In other words, polar bears are in great danger.

    If you travel by air across the center of Africa or South America, you fly over forests for thousands of kilometres．These great forests are the seas of trees．They are full of thousands and thousands of different kinds of plants and animals．However, the world's forests are getting smaller all the time．We are cutting down the trees because we need wood, and we need more farmland as well．Some people say that there will not be any forests like these left in 20 to 30 years．What will happen if they disappear? If we cut down our forests, a lot of plants and animals will disappear from the world, in a lot of places the new farmland will soon look like the old deserts, for example, crops will not grow there．What's worse, it will not rain very often, and the weather will get very hot．Perhaps the climate of the world will change, which will be dangerous for everyone in the world．That is why we must take care of our forests．

    Do you know when Earth Day is? It is on April 22nd, and it began in the US in 1970. Earth Day in our country began in the 1990s. Earth Day is a good time for people to think about what they can do to make the Earth clean and beautiful, because now the environment on Earth is getting worse and worse. In fact, there are many things we can do to protect our Earth.

    Plant trees.

    Planting more trees helps to reduce greenhouse gases (温室气体), stop the earth from running away and provide homes for animals.

    Don't waste.

    Do not waste water. Use both sides of paper when you write. Turn off the light when you leave. Take your cup with you. Take your own shopping bags to the supermarket. Stop using the plastic bags. Don't use any disposable things. We should save energy.

    Don't throw rubbish in the streets.

    When rainwater washes over the rubbish, it becomes polluted. The polluted water flows into the rivers and seas.

    Ride your bike or take a bus.

    Too many cars pollute the air and waste energy.

    Remember: Every day is Earth Day.

    We should protect the Earth every day, not just on Earth Day. The Earth is our home. Let's do our best to make the Earth cleaner and more beautiful.

    On July 20, 1969, Neil Armstrong became the first person to walk on the moon. The space suit Armstrong wore on his lunar mission has come to symbolize courage and human achievement. In 1971, the Smithsonian's National Air and Space Museum(NASM), in Washington, D.C., acquired (获得) Armstrong's suit. It stood proudly on display for visitors to admire, until there were changes in the suit's appearance. "Over time, the suit started to show signs of deterioration," Lisa Young said. Young is an objects conservator (文物修复员) at NASM. It is her job to preserve(维护)historical and cultural objects.

    "Plastics are tricky to conserve," Young says. They tend to degrade (降解)over time. Neoprene is one type of plastic in Armstrong's suit. The material can turn brittle and break into tiny pieces. This would ruin the space suit. NASM, seeking ways to preserve this historical treasure, removed it from display in 2006.

    What causes plastic to degrade in the first place? "Many things," Odile Madden says. Madden is a scientist in California. She explains that sunlight, water, humidity, and dust can cause plastics to degrade. "Some plastics will break down no matter what we do," Madden says.

    Yet history can still be saved. Scientists have found that storing plastics in cooler temperatures, with lower humidity levels, can slow degradation. Young says Armstrong's suit is currently in storage at a temperature of 63°F and a lower relative humidity of 30%. These conditions will be maintained when Armstrong's suit returns to a museum display case in time for the 50^th anniversary of the moon landing.

    Plastic degradation affects art, too. Artist Claes Oldenburg created False Food Selection in 1966.It consists of plastic food displayed in a wooden box. The plastic food used to look real. Now, some of it has flattened and yellowed. Georgina Rayner is a conservation scientist at Harvard Art Museums, in Cambridge. She says the artwork's wooden box produces an acidic (酸的) gas that eats away at the plastic. This speeds up the degradation process. But Rayner is motivated to conserve this artwork and others like it. "Plastics are a part of our history," she says. "It's important to preserve them so that future generations can understand the journey we've taken."