Water travels from sea to land and then returns to the sea recurrently. The whole process includes the following phases:
1.When the sun shines on the ocean causing the temperature of seawater to rise, part of the water evaporates and becomes an invisible gas called vapor, which is then mixed with the air.
2.As the vapor rises higher and higher in the sky, it meets cold air and as a result condenses into small droplets of water.
3.Myriads of these droplets gather to form clouds, which are then blown to the sky over land by wind.
4.Droplets in the cloud turn into rain, snow or hail, fall to the ground and turn into surface water. Part of the surface water then seeps into the ground while flowing on it.
5.Finally the remaining surface water comes into rivers and streams and flows back into the sea again, marking the end of one cycle and the start of the next.

People living by the sea know that seawater rises and falls twice every day regularly. We name this natural phenomenon as tide. Sometimes you may wonder what causes such perpetual motion. Well, here is the answer. When the earth and the moon are rotating constantly in space, the moon will exert a gravitational force on the seawater on the earth. Meanwhile, the earth is revolving by itself all the time. As a rule, seawater will rise when it is facing the moon and rise again when it is opposite the moon. Therefore, we get two high tides with two low tides daily.

Rivers flow down the slopes of mountains and empty into the open sea. As the river approaches the sea, it becomes wider and wider and the flow becomes slower and slower. The life of a river is pretty like that of a person. A river flowing from a mountain into the sea is just like a person growing from young to old. The whole process can be divided into the following stages.
1) Adolescent river: Some rivers originate from the melting edges of icy mountains or glaciers, while others from lakes and streams on the surface of land. A river originating from mountains rushes down mountain slopes rapidly, through cliffs and to deep valleys.
2) Mature river: After leaving its birthplace in the mountains, the river continues to wind its way through plains, often branching into many tributaries.
3) Old river: The river flows lazily through a flat area, called delta, near the sea and finally empties into the sea at the river mouth.

Although we rely on the world’s oceans for food, we treat them like rubbish bins. Waste is pumped and dumped into the water while pesticides and other man-made pollutants are washed into the ocean by rivers and streams. The pollution of the world’s oceans is harmful. Many sea animals are injured, strangled or suffocated each year because of floating debris called flotsam. The high level of toxic waste in a few seas is poisoning some animals and driving others away.

They are Pacific, Atlantic, Indian, Southern and Arctic Oceans. They have a total sea area of 38 million square kilometers. The Southern Ocean occupies about 20% of the surface area of the global ocean. A decision by the International Hydrographic Organization in the spring of 2000 delimited a fifth world ocean - the Southern Ocean. It is now the fourth largest of the world's five oceans (after the Pacific Ocean, Atlantic Ocean, and Indian Ocean, but larger than the Arctic Ocean).The Southern Ocean is deep, 4,000 to 5,000 meters over most of its extent with only limited areas of shallow water the Antarctic continental shelf is generally narrow and unusually deep, its edge lying at depths of 400 to 800 meters. The Southern Ocean is notorious for having some of the strongest winds and largest waves on the planet. It is also home to the largest current (The Antarctic Circumpolar Current) in the world ocean. The Circumpolar Current carries between 135 and 145 million cubic meters of water per second from west to east along a 20,000 km long path around Antarctica. The flow of the Circumpolar Current is equivalent to about 150 times the flow of all the world’s rivers combined.

Most rivers in China flow from west to east into the Pacific Ocean (except a few in the Southwest that flow to the South). The Yangtze River (Changjiang) which originates from the Tanggula Mountain on the Qinghai-Tibet Plateau and flows into the East China Sea, is 6,300 kilometers long and is the largest river in China. It is the world’s third longest river after the Nile and the Amazon. The Yellow River (Huanghe) which is 5,464 kilometers long and flows into the Bohai Sea, is the second longest river in China. Both rivers are the birthplace of ancient Chinese civilization with a wealth of historical sites and relics many of them buried underground.

If you had ever swam in the sea, you would have surely tasted seawater and know it is salty. The reason why seawater is salty is that it contains a variety of minerals and salt is one of them. Minerals and salts in the water are usually dissolved from rocks at the bottom of the sea. Furthermore, various minerals from rocks on land are carried into the sea by vapor or rivers. Water from rivers do not taste salty and is therefore termed fresh water. If you swim in the ocean, the salt content of the water will create more buoyancy for you so that you can float on the surface easily. In some areas water can be very salty. For instance, the Dead Sea adjacent to Israel and Jordan contains the saltiest water in the world. In reality, the Dead Sea is a lake rather than a sea. In other areas, however, seawater may be less salty due to the lower concentration of salt. The 'freshest' seawater is located at the Atlantic Ocean near South America. This is caused by the dilution of fresh water from many rivers, one of which is the giant Amazon River dumping millions of tons of it into the ocean every minute.

Icebergs
Water near the surface is relatively warm due to direct sunlight. In the tropical areas near the equator, the sun shines for a long time every day. Therefore, the water there always stays nice and warm so that people can swim in it all year round. However, in the Arctic and Antarctic areas, the sun shines for a short time. As a result, in the seas close to the Arctic Ocean and the Antarctica, water normally stays frozen. Huge ice blocks floating on the surface of the sea are called floes or icebergs.
Shorelines
A shoreline is a boundary between the sea and the land. Constant attacks of waves and winds from the sea eat into the land along the shoreline bit by bit, reducing the shoreline into a variety of irregular shapes. Due to repeated erosions, the shape of shorelines has been changing all the time. Rocks in cliffs vary in hardness.Some are soft and some are hard. As time goes by, soft rocks are worn away gradually and cave in, forming a concave area called erosion platform, whereas hard rock stick out and grow into a peninsula or a cape called sea cliff. Some sea cliffs become sea erosion arch bridges after their central parts are eroded through. Some sea cliffs become sea stacks after most parts of them are washed away with only small stacks left. Along some shorelines, there are small cavities, called blow holes, at the bottom of the cliffs, which are also 'drilled' by waves from the sea.

The ocean current or the 'river in the ocean' is due to water flowing in a certain direction on a large scale in the ocean. It brings seawater from one place to another. Some ocean currents circulate only in shoal water zones. For instance, in Polar Regions, icebergs float along with surface currents. Other ocean currents flow in deep water along seas. All over the world there are many surface ocean currents. Some are warm currentsand some are cold currents. The warm current has a higher temperature and the cold current has a lower temperature compared with surrounding seawater. All the above currents are grouped into six major oceanic circulation systems. In the tropical regions close to the equator, strong winds tend to push surface currents to flow from east to west. On the contrary, in the Polar Regions wind circulation causes the opposite flow of the surface current. When the ocean current reaches the land, it will change direction and flow along the seashore.

Lakes are depressions in the ground with a large body of water. Most lakes have a river flowing through. The river brings a lot of mud and sand into the lakes. As a result, some of the lakes disappear completely after they are filled up with mud and sand. Most waterfalls hang on mountain slopes. A waterfall usually appears where a river winds through a cliff. The rocks of the cliff are so hard that they are very difficult to wash away by water, so the river dashes down to form a waterfall.

Millions of years ago, there was only one ocean on the earth. Thereafter, the great ocean was divided by land into five parts, namely the Pacific Ocean, the Atlantic Ocean, the Indian Ocean, the Arctic Ocean and the Southern Ocean. The sea is just a part of the ocean. On the earth there are numerous seas, among which the Mediterranean Sea, the Bering Sea and the Caribbean Sea are the largest.
Sometimes an ocean is also called a sea. There are millions upon millions of animals living in the ocean. Fishermen catch marine animals and provide us with nutritious and tasty seafood. The ocean also abounds with other useful treasures. For example, engineers extract oil and natural gas from seabed rocks. Many volcanic eruptions take place in the ocean. Some volcanoes are so high that they emerge from the water, forming new islands. Moreover, floating in the ocean are some huge blocks of ice, called icebergs. Initially, they were part of the icebergs on the coasts and later floated into the ocean. Some icebergs are even as huge as islands. At the edge of all continents, the land extends into the sea to form the underwater shallow continental shelf. The edge of the continental shelf is like a giant steeple extending sharply down to the seabed. On the seabed are large expanses of flatland, called abyssal plains. The seabed is not always flat everywhere, for some places covered with small extinct volcanoes are abyssal hills. On the seabed there are many towering mountain ridges, also called Sea Ridges left behind by strong volcano eruptions. There are also deep and serene gorges called seafloor trenches on the seabed. Like the famous Colorado Grand Canyon, the underwater topography is very magnificent and colorful.

Although most fish need a steady supply of water across their gills in order to breathe, some species are able to survive out of water for as long as several hours. One such creature, found mainly in the mangrove swamps of Southeast Asia, is the mudskipper. Not only can it breathe air, but it can also skip across muddy shores-hence its name-and even shin its way up tree trunks. Out of water, the 12 centimeter long, bug-eyed mudskipper absorbs life-sustaining oxygen through its skin and gill chambers.

Although saltwater fish drink great quantities of water, freshwater fish drink almost none at all. The difference stems from the creatures’ need to maintain the proper balance of salt and water in their bodies. The oceans are three times saltier than the body fluids of the fish that live there. Because of the natural process known as osmosis, the water inside a saltwater fish tends to flow through its skin and gills into the sea water outside. To replace this lost liquid, the saltwater fish must swallow huge amounts of sea water. In freshwater fish, whose bodies have a higher concentration of salt than the waters around them, the process occurs in reverse. Freshwater fish constantly absorb water into their bodies, so they have no need to drink it; instead, they discharge the excess fluid by copious urination

Like almost all living creatures, fish need oxygen in order to exist. Most of them obtain this vital element through special sieve-like organs that are called gills. Located just behind the mouth cavity on both sides of the head, the gills are usually covered by a flap known as the operculum. Underneath the operculum are four overlapping rows of blood red gills. Each gill consists of a bony arch that supports numerous gill filaments-pairs of slender, fleshy appendages that resemble closely spaced teeth on a comb. Protruding from each filament are tiny membranes, or lamellae are so thin that blood flowing through them extracts oxygen from water passing across the gills. As part of the same process, the lamellae remove carbon dioxide from the blood and expel it into the water. Water contains 1/30 as much oxygen as air does, and this gas exchange –oxygen in, carbon dioxide out- is a key component of aquatic survival.

Deep-sea fish live under great pressure. At 900 meters below the surface, the water pressure is 100 times that of sea-level air. In the deepest ocean-some 11,000 meters down-every square centimeter of water i sunder more than one metric ton of pressure. Such pressures would crush a land animal, yet marine creatures are unaffected by them. This is because their body tissues are filled with liquids and dissolved gases that are just as compressed as the surrounding sea water. The internal fluids of deep-sea fish therefore push outward as forcefully as the ocean presses in. Fish with gas-filled swim bladders are especially susceptible to injury from rapid changes in pressure.

Scientists define a fish as a cold-blooded aquatic vertebrate. This means its blood temperature matches that of its surroundings, it lives in water, and it has a backbone. Most fish species breathe with gills, reproduce by laying eggs, and are covered with protective scales. A number of specialized physical features make fish well adapted to aquatic life. Fish have developed with light, flexible skeletons and are nearly weightless in water. Attached to this framework are strong muscles that power the fish through the water. Fins-wing like structures on the back, belly, sides, or tail-act as rudders, stabilizers, or paddles. The internal organs, including the important swim bladder, are in the body cavity beneath the vertebral column.