Pains and Sickness
Headaches can be very painful and can last for a long time.
If you have a headache, your head hurts.
A sprained ankle(脚踝扭伤) can be so painful that you can't walk.
Someone with a sprained ankle may need to use crutches(crutch的复数，拐杖) to walk.
If you have a sore(发炎疼痛的）throat, it may be painful to swallow(吞，咽).
A sore throat can be caused by viruses(virus的复数，病毒) or bacteria(bacterium的复数，细菌).
A burn is an injury to the skin caused by fire or heat.
If you get too close to a fire, you will be burned.
An earache is a sharp, dull or burning pain in the ear.
Common causes of an earache include an infection, a change in air pressure or water in the ear.
Landforms（地形，地貌）
Mountains are landforms higher than 2,000 feet, or about 600 meters, above sea level.
Hills are like mountains, but not as tall.
Mountains are formed by forces deep within the Earth and are made of many types of rocks.
Rivers are streams of water that usually begin in mountains and flow into the sea.
Many early cities were built alongside rivers.
Many early civilizations developed near rivers.
Most of the world's water is in the oceans, which are large bodies of salt water.
The oceans are also an important source of oxygen.
Desserts are areas of land that receive little or no rainfall.
The amount of rainfall in some deserts is so low that only a few plants can survive.
Dessert temperatures can range from cold at night to very hot during the daytime, often above 120 degrees Fahrenheit.
A valley is a low, relatively flat area between hills or mountains, often with a river running through it.
Some valleys are formed by flowing water, and others are formed by glaciers(冰川).
Science
A force is a push or a pull on an object.
A force changes the motion of a moving object.
A force can be strong or weak, and it always has a direction.
The Earth is a planet that goes around the sun in a circular orbit(圆周轨道，圆形轨道).
The speed of its motion doesn't change, but the direction of its motion does.
The Earth has a layer of gas above its surface, which helps to support life.
Atoms are the building blocks(积木) of matter.
At the center of an atom is its nucleus(核心), which has a positive electric charge(正电荷).
Outside the nucleus are electrons(电子), which have a negative charge(负电荷).
Asteroids(小行星) and comets(彗星) are objects that orbit(绕…轨道而行) our sun but are smaller than planets.
Asteroids that pass close to the Earth are called near-earth objects(近地目标星体，近地物体).
The layer of gases just above Earth's surface is our atmosphere(大气层).
The atmosphere is made up of gases, such as oxygen and nitrogen (氮).
Without an atmosphere, there would be no life on Earth.
Geometry
The two lines on the left intersect(相交) each other at a right angle, which is 90 degrees.
The two lines on the right are parallel, which means they never cross.
The adjacent(相邻的，邻近的) sides of a rectangle are perpendicular(垂直的), which means they meet at a 90-degree angle.
The opposite sides of a rectangle are equal and parallel, which means they never cross.
The area of a rectangle is the length times the width.
A circle is a two dimensional figure(二维图形，平面图形) with a radius and circumference(周长), which is the distance around the circle.
The area of a circle is pi r squared, where r is the radius and pi is the ratio of a circle's circumference to its diameter.
A sphere is a three dimensional figure(三维图形，立体图形) with a radius and a volume, which is 4/3 pi r cubed.
All points on the surface of a sphere are equidistant(等距离) from its center.
A right triangle(直角三角形) is a two dimensional figure with 3 sides, two of which are perpendicular.
To find its area, multiply the lengths of the two perpendicular sides, a and b, and divided by two. A = 1/2 ab.
Dental Problem
A dental cavity(蛀牙) is a hole in a tooth.
A cavity is caused by bacteria(细菌) in the mouth.
Brushing your teeth frequently and regularly can help prevent cavities.
A cracked tooth(隐裂牙) can be caused by biting down on something hard.
Dentists treat a cracked tooth by covering it with a crown(牙冠，齿冠).
A dental x-ray is a picture of your teeth and gums(牙龈).
An x-ray shows what can't be seen on the surface of the teeth and gums.
Braces(牙套) are used by dentists to straighten teeth over a period of years.
Though braces are usually for children, they can also be used for adults with crooked teeth(歪牙，不整齐的牙齿).
To extract a tooth(拔牙), means to remove it from the mouth.
Dentists have to pull hard to extract a tooth.
Internal Organs
The heart pumps blood to all parts of the body through a system of arteries and veins(动脉和静脉).
It's located in the chest, between the lungs, and it beats about once per second.
The kidneys(肾脏) remove waste and toxins(毒素) from the blood.
They are bean-shaped organs and are about 10cm long and 6cm wide.
The lungs exchange oxygen from the air with carbon dioxide from the blood.
They expand when inhaling(吸气) air, and contract(收缩) when exhaling(呼气) air.
The stomach is below the heart, and it's part of the digestive system.
It uses acids(酸) to digest food before passing it to the small intestine(肠).
The bladder(膀胱) is a balloon-shaped organ that collects urine(尿) from the kidneys.
In an adult, the bladder can hold about half a liter(公升) of urine.
Contemporary People (当代人)
Co-founder(联合创始人) of Apple, Steve Jobs was a major figure in the computer industry.
He was self-centered(以自我为中心的) and difficult to work for, but he led the development of many great products.
He died of cancer when he was just 56 years old.
Jack Ma, is one of the richest and most famous men in China.
He comes from a poor family and failed his college entrance exams(高考) twice.
Despite his many failures, he has built one of the world's largest Internet companies.
David Beckham is one of the world's richest and most famous athletes.
He started playing professional football when he was just 17.
Much of his success as an athlete is because he practiced harder than many other players.
As a young girl in Pakistan, Malala became famous for her writings on women's education.
In 2012, She was shot in the head and nearly killed.
She survived the injury and became the world's most famous teenager.
Born in 1942, Stephen Hawking was one of the world's most famous scientists.
When he was 23, he was diagnosed with(被诊断出) ALS, a rare disease of the nerves.
Because of his disease, he was unable to walk and talk.
Personality Types
Shy people are usually quiet and uncomfortable around other people.
They generally dislike going to social events such as parties.
It's often difficult to make friends with a shy person because they won't open up(敞开心扉).
Outgoing people enjoy being with others and are comfortable in social situations.
They are friendly and pleasant to be with, though sometimes they can't be trusted.
Self-centered people like to be the center of attention.
They like people to look at them, and pay attention to them.
Self-centered people often talk too much, and don't listen enough.
Selfish and greedy people don't share things with others.
They keep things for themselves and always want more.
People who are responsible and trustworthy(可靠的) are important in almost any situation.
They can be depended on to get things done.
These people can be trusted.
