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.
America’s founders did debate a bit as to whether or not to force children to attend schools, and they decided to leave such decisions to individual families and local and state governments. The words “education” and “school” do not appear in any of our founding documents, such as the Declaration of Independence, the Constitution, or the Bill of Rights.
Some of our most famous inventors, writers, and politicians were self-taught, learning through mentoring or apprenticeships, conversation, and reading. In 1850, Massachusetts became the first state to institute a compulsory schooling law.
When the time came to honour the patron saint of Ireland’s birthday, church officials gathered solemnly to choose a day, then realized that most of St. Patrick’s life was a mystery. They finally narrowed his birthdate down to either March 8 or 9, but because they couldn’t agree which was correct, they decided to add the two together and declared March 17 to be St. Patrick’s Day.
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.
Carved into the southeast face of a mountain in South Dakota are the faces of four presidents: George Washington, Thomas Jefferson, Theodore Roosevelt, and Abraham Lincoln. Known as Mount Rushmore, these 68-feet (20.7 meters) high granite sculptures were the brainchild of South Dakota state historian Doane Robinson. In 1923, he conceived the project to attract more people to the Black Hills of South Dakota.
Congress passed legislation that authorized the carving in what is today known as Black Hills National Forest. In 1927, the sculptor Gutzon Borglum and 400 workers began the project, using dynamite to remove unwanted rock. Mount Rushmore was completed in 1941.
A cord is a traditional unit of volume used to measure stacked firewood. In the United States, the cord is defined legally as the volume of a stack of firewood 4 feet (1.2 meter) wide, 8 feet (2.4 meter) long, and 4 feet (1.2 meter) high. (In Maryland, the law specifies that the wood be stacked “tight enough that a chipmunk cannot run through it.”) The name comes from an old method of measuring a stack of firewood using a cord or string.
In the U.S. timber industry, the cord is also used as a unit of weight for pulpwood. The weight varies with tree species, ranging from about 5,200 pounds (2,358 kilograms) for pine to about 5,800 pounds for hardwood.
The weight of a carat (200 milligrams), the standard unit of measurement for gemstones, is based on the weight of the carob seed, which was once used as a weighing standard by jewelers in Africa and the Middle East. Historians believe the word “carat” is derived from an Arabic word meaning “bean” or “seed.”
In a way, yes. Before the invention of refrigeration machines in 1805, people cut ice from ponds in winter and stored it in an icehouse. These icehouses were pits dug into the ground and lined with wood or straw and packed with snow and ice.
They were covered with an insulated roof. Because cold air sinks, the pit remained very cold, keeping foodstuffs cold and unspoiled until the early summer months.