Can you imagine a sea of sand three times bigger than India? This is the Sahara Desert. It covers 11 countries in North Africa and is over 9 million square kilometers in area. That’s more than 25% of Africa.

In the Sahara, temperatures are very different during the day and at night. During the day, the hottest time is between 2 p.m. and 4 p.m., when temperature rises to 33°C. But it is very cold at night —the coldest time is at 4 a.m., when temperatures fall to -1C. The Sahara is very dry. The average precipitation (降水) in a year is only 70 millimeters.

Because of the extreme temperatures in this desert, it is a very difficult place to survive. Marco Rivera has some tips.

Take warm clothes and a blanket. During the day, cover your body, head and face. Clothes protect you from the sun and keep water in your body. You will also need a warm blanket at night. It can get cold very quickly. When the temperature drops, it can be a shock and make you feel even colder.

You can use your car tires (轮胎) to make a fire. A fire is easy to see. It will help people find you and it will keep you warm at night.

Try to drink some water once every hour. You need your water to last as long as possible. Drink only what you need. When you talk, you lose water from your body. Keep your mouth closed and do not talk.

If you eat, you will get thirsty and drink all of your water more quickly. You can eat a little, but only to stop you from feeling very hungry. Eat very small amounts of food and eat very slowly. You can live for three weeks with no food, but you can only live for three days if there’s no water.

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Everyone dreams of changing the world. But can teenagers really make a difference? And how can they go about it?

Michael Norton's book 365 ways to Change the World is a good starting point: The book gives an idea or task for every single day of the year. The tasks include offering to work at a local homeless shelter(护所)and planting tree.

The book says that one simple act, no matter(无论)how small, can help someone in need. It may even influence(影响)other teens to do something Nathan Anthony, a student at Maple High School in Modesto, California, US, agrees. "It is impossible to get anything in life without giving something to someone or working hard at school or workplace, Anthony said.

Another Maple student, Ariane Mota, has the same idea. "Giving is our way of repaying those who help us, she said.Whether(无论) it is a helping hand or a donation(捐赠) we should others to make their day or life better."

But how often do teens see peers(同龄人)helping a stranger or doing something nice for someone?

According to Anthony, not lot. He and Mota both believe this needs to be changed "I help my friends by telling them that their lives are better than those poorer people，" Anthony said. "Humor is also a great tool."

So whether you're helping to find a cure(疗法 )for bad illness or simply smiling at a stranger, do something every day to help change the world for the better.