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Invisibility Ring
Scientists can"t yet make an invisibility cloak like the one that Harry Potter uses. But， for the first time， they"ve constructed a simple cloaking device that makes itself and something placed inside it invisible to microwaves.
When a person "sees" an object， his or her eye senses many different waves of visible light as they bounce off the object. The eye and brain then work together to organize the sensations and reconstruct the object"s original shape. So， to make an object invisible， scientists have to keep waves from bouncing off it. And they have to make sure the object casts no shadow. Otherwise， the absence of reflected light on one side would give the object away.
Invisibility isn"t possible yet with waves of light that the human eye can see. But it is now possible with microwaves. Like visible light， microwaves are a form. of radiant energy. They are part of the electromagnetic spectrum， which also includes radio waves， infrared light， ultravioletmys， X rays， and gamma rays. The wave lengths of microwaves are shorter than those of radio waves but longer than those of visible light.
The scientists" new "invisibility device" is the size of a drink coaster and shaped likearing. The ring is made of a special material with unusual ability. When microwaves strike thering， very few bounce off it. Instead， they pass through the ring， which bends the waves all the way around until they reach the opposite side. The waves then return to their original paths.
To a detector set up to receive microwaves on the other side of the ring， it looks as if the waves never changed their paths as if there were no object in the way! So， the ring is effectively invisible.
When the researchers put a small copper loop inside the ring， it， too， is nearly invisible.
However， the cloaking device and anything inside it do cast a pale shadow. And the device works only for microwaves， not for visible light or any kind of electromagnetic radiation. So， Harry Potter"s invisibility cloak doesn"t have any real competition yet.
Harry Potter is mentioned in the passage， because scientists __________. 查看材料
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According to the passage， dairies are __________. 查看材料
The amount of water your body needs per day is related__________.
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The Drink Your Body Needs Most
（1） Our bodies are estimated to be about 60% to 70% water. Blood is mostly water， and our muscles， lungs， and brain all contain a lot of water. Water is needed to regulate body temperature and to provide the means for nutrients （滋养物） to travel to all our organs. Water also transports oxygen to our cells， removes waste， and protects our joints and organs.
（2） We lose water through urination （排尿）， respiration （呼吸 ）， and by sweating. If you are very active， you lose more water than if you do not take much exercise. Symptoms of mild dehydration （脱水） include chronic pains in joints and muscles， lower back pain， headaches，and constipation （便秘）. A strong smell to your urine， along with a yellow color indicates that you are not getting enough water. Thirst is all obvious sign of dehydration and in fact， you need water long before you feel thirsty.
（3） A good rule of thumb （好的做法） is to take your body weight in pounds and divide that number in half. That gives you the number of ounces （盎司） of water per day that you need.
For example， if you weigh 160 pounds， you should drink at least 80 ounces of water per day.
If you exercise you should drink another 8ounce glass of water for every 20 minutes you are active. If you drink coffee or alcohol， you should add at least an equal amount of water. When you are traveling on an airplane， it is good to have 8 ounces of water for every hour you are on board the plane.
（4） It may be difficult to drink enough water on a busy day. Be sure you have water handy at all times by keeping a bottle for water with you when you are working， traveling， or exercising. If you get bored with plain water， add a bit of lemon for a touch of flavor. There are some brands of flavored water available， but some of them have sugar or artificial sweeteners that you don"t need.
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