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      There are too many cities around the world. And each city needs water in many ways. Water on Earth falls mainly as rain. So the rainfall (降雨量) is important to each part of the world. And it is different from one city to another. The following chart shows the average (平均的) rain fall in four different cities in July and January.

    Energy is everywhere. We use energy every day.

    Electricity is a form of energy and this is what powers the TV, hairdryer, phone, fridge, electric toothbrush and more! Our growing need for energy is one of the problems facing the world today, because producing electricity also produces pollution.

    Here are some simple ways to make a big difference when using electricity.

On or off? Leaving televisions and computers on with the little red light showing still uses electricity, even though they might look like they're turned off. To stop this you can turn them off.

    Lights out! Turning lights off when you're not in the room can save a lot of energy.

    Fridge door closed. The fridge is an important part of the kitchen, because it keeps food fresh, cool and healthy! But it takes a lot of energy to keep cool, so try not to leave the fridge door open or put hot things inside.

    Use the sun's energy. An increasing number of people are fitting solar panels（太阳能电池板） to their houses. They create electricity from sun-light, which can then be used to power our homes. Solar heating panels make hot water, reducing the need for electricity or gas.

    There will be a kind of new cars in the future. People will like this kind of small cars better than the big ones. The car is as small as a bike. But it can carry two people in it. Everybody can drive it easily, just like riding a bike. Even children and old people can drive them to schools or parks.

    If everyone drives such cars in the future, there will be less pollution in the air. There will be more space for all the cars in cities, and there will also be more space for people to walk in the streets.

    The little cars of the future will cost less money to buy and to drive. These little cars can go only 65 kilometers an hour, so driving will be safer. The cars of the future will be fine for going around the city, but they will not be useful for a long trip.

    This kind of cars can save a lot of gas. They will go 450 kilometers, then they have to stop for more gas. They are nice cars, aren't they?

    Don't be too surprised if you see a car going down the road without a driver in Beijing these days. It might be one of Baidu's self-driving cars. Last month, Baidu was granted (授予) a license to test its self-driving cars on open roads in Beijing, China Daily reported.

    Self-driving cars have been in development since 2010, when Google announced that it was making such cars. Since then, companies like Tesla, Uber and Baidu have been working to produce and test their own self-driving cars.

    Self-driving cars are expected to revolutionize (革命) the way we get around. They are safer and more environmentally friendly than regular cars. They can remove the stress of driving and allow disabled or blind people to get around more easily.

    But self-driving cars have their own flaws (缺陷) as well. In one test, researchers pointed a laser pointer (激光棒) at the car's sensors (传感器) while it was moving. The car mistakenly thought that the laser beam was in its way and slowed down or changed direction. Flaws like this could be used by people to trick self-driving cars, according to the Guardian.

    The software that is used to direct the cars may not be safe enough, reported Live Science. It may crash (崩溃). When we use our computers or smart phones, they sometimes crash without any reason. But if this happened to a self-driving car, the results could be deadly. Improvements to the software could be a solution to this problem.

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