Today, about 3.5 percent of ocean water is salt. When planet Earth was still young, its atmosphere contained a mix of hydrogen chloride, hydrogen bromide, and other gasses from volcanoes. Oceanographers (scientists who study the ocean) believe that some of these gases dissolved in the early ocean, making it salty.
Today, however, most of the salt in the oceans comes from rain. Rain falling on the land dissolves the salts in eroding rocks, and these salts are carried down the rivers and out to sea. The salts accumulate in the ocean as water evaporates to form clouds. The oceans are getting saltier every day, but the rate of increase is so slow that it hard to measure. If the oceans suddenly dried up, there would be enough salt to build a 180-mile- (290-kilometer) tall wall around the equator.
Frogs are able to make their croaking noises because they have simple vocal cords that have two slits in the bottom of the mouth. These slits open into what is called a vocal pouch. When air passes from the lungs through the vocal cords, a sound is produced. The inflating and deflating vocal pouch makes the sound louder or quieter.
That sound changes depending on the kind of frog there are as many different kinds of croaks as there are frogs! Frogs croak for the same reasons that many animals make noises: to track down and then select a mate, and to protect their territory from other male frogs.
Earth is more active, in terms of both geology and weather, which makes it hard for craters to remain. Even those craters scientists can see on the surface—which may be millions of years old—have been overgrown by vegetation, weathered by wind and rain, and changed by earthquakes and landslides. The Moon, meanwhile, is geologically quiet and has almost no weather, so its hundreds of thousands of craters are easy to see.
The craters are the result of both meteorites and volcanic activity. Interestingly, some of the oldest Earth rocks might be awaiting discovery on the Moon, having been blasted there billions of years ago by asteroid impacts that shook both worlds.
The brain is the body’s command center; everything we do—eating, talking, walking, thinking, remembering, sleeping—is controlled and processed by the brain. As the most complex organ in the human body, the brain tells us what’s going on outside our bodies (whether we are cold or hot, for instance, or whether the person we see coming toward us is a friend or a stranger) as well as what’s going on inside our bodies (whether we have an infection or a broken bone, or whether we feel happy or sad).
The key to the body’s nervous system, the brain contains between 10 billion and 100 billion nerve cells, or neurons. Neurons combine to form the body’s nerves, thin cords that spread from head to toe and all parts in between. Neurons take in and send out electrical signals, called impulses, that control or respond to everything your body does and feels. The brain is like a very busy, high-speed post office, constantly receiving messages and sending them out all the time; it handles millions of nerve impulses every second.
At least two ancient Greek athletes would have done well in the modern games; their Olympic records stood until the twentieth century. Twenty-six hundred years ago, an athlete named Protiselaus threw a cumbersome primitive discus 152 feet from a standing position. No one exceeded that distance until Clarence Houser, an American, threw the discus 155 feet in 1928. In 656 BC, a Greek Olympian named Chionis leapt 23 feet, 1.5 inches, a long jump record that stood until 1900, When an American named Alvis Kraenzlein surpassed it by 4.5 inches.
Jupiter, Saturn, Uranus, and Neptune have rings—or thin belts of rocks—around them. Jupiter’s ring is thin and dark, and cannot be seen from Earth. Saturn’s rings are bright, wide, and colorful. Uranus has nine dark rings around it, and Neptune’s rings are also dark, but contain a few bright arcs.
At one time all of the planets, Earth included, had rings. These rings were unstable and the material was either lost in space or collected into the satellites of these planets.
Oxygen is necessary for all humans, animals, and plant life to survive. When Earth was first formed, its atmosphere had no oxygen—the colorless, odorless and tasteless gas that makes up about 20 percent of the air we breathe. It had only a deadly combination of hydrogen, methane, ammonia, and hydrogen cyanide.
The hydrogen escaped into space and ultraviolet radiation from the Sun broke down the mixture, leaving only nitrogen and carbon dioxide. Only when life began and photosynthesis (the conversion of light energy into chemical energy by living organisms) occurred did oxygen first appear—about 3.4 billion years ago.
Yes. Not all plants are seed plants. Some plants, such as ferns and mosses, reproduce with spores instead of seeds. Spores, like seeds, can survive harsh conditions and develop into new plants. However, unlike seeds, spores are produced without fertilization and contain neither a plant embryo nor endosperm. Some plants can reproduce without spores or seeds through vegetative reproduction, in which a part of the stem or root gives rise to a new plant.
Yes. Organic farmers also try to do more tasks using human power rather than gas-powered vehicles, thereby using less fuel and cutting down on pollution. Organic farms that raise livestock like dairy cows or chickens feed the animals with natural food, avoiding pollution-causing chemicals and growth hormones that make cows produce more milk and chickens produce more eggs.
Some organic farmers also allow their animals to roam in a large area (such animals are described as “free range”) rather than keeping them in small, climate-controlled pens for their entire lives.
Yes. Mars, the fourth planet from the Sun, is called the Red Planet. It looks red because the rocks on the surface contain rusted iron. It has an atmosphere with clouds, winds, and dust storms—its red dust floats in the atmosphere and gives the planet a red sky.
Mars, which has two moons, orbits the Sun every 687 days and rotates on its axis once every 24 hours and 37 minutes.