There is no way specific way to place an order for a baby brother or a baby sister. The gender (boy or girl) of a baby is determined by whether the father’s fertilizing sperm has an X or a Y chromosome. An X chromosome will lead to a girl, and a Y to a boy. (Mothers always contribute an X chromosome.) Although scientific methods are available to help parents organize their chromosomes and take advantage of the fact that the “boy” sperm has less DNA than “girl” sperm, they can be expensive and unreliable. One method, called the Shettles method, recommends that if parents want a girl, they should plan to make a baby right around the time of ovulation.
At this time, the egg is as far away as possible from the incoming sperm so the long-distance runners of the sperm world, the X sperm, have a better chance of making it to the egg. For a boy, the method suggests that parents plan to make a baby about two to four days after ovulation. That way, the short-distance sprinting Y sperm can make it to the egg first. Many doctors say that although this method is based in science, it is no guarantee that a couple will have a baby boy or a baby girl.
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.
A cobweb is an old, abandoned spider web that has collected dirt and dust. Sometimes the cobwebs you see in ceiling crevices and along floorboards are several draglines that spiders no longer use.
The common house spider—which feeds on many insects daily—often abandons webs that do not yield prey, and then constructs new ones until it finds a productive site. It’s best to sweep these old cobwebs away, and let your house spiders spin new webs, preferably outdoors!
Airplanes function according to a complex mix of aerodynamic principles—theories that explain the motion of air and the actions of bodies moving through that air. Airplanes get their power from engines. Small planes generally use piston engines, Which turn propellers that push aircraft through the air in the same way that boat propellers push vessels through water. But bigger planes use jet engines, powered by burning fuel. These engines expel great amounts of air that thrust a plane forward and up. An airplane must be in constant motion—its wings slicing through rushing air to create lift—in order to stay up; moving air is also required to steer it. In order to get enough lift to rise into the air on takeoff, an airplane has to travel along the ground first at great speed.
Airplanes are able to lift into the air and stay there because of the shape of their wings. An airplane wing is flat on the bottom and curved on the top. When a plane’s engines push it forward, air divides to travel around its wings. The air that passes over the larger curved top moves faster than the air that passes under the flat bottom. The faster-moving air on top becomes thinner and has lower pressure than the air below, which pushes the wing up. Uneven air pressure caused by the shape of an airplane’s wings, then, creates a force called lift, which allows an aircraft to fly.
Bonsai means “tree in a pot” in Chinese and is pronounced BONE-sigh. It is a special method for growing trees or shrubs in a pot, tray, or dish, which results in an artificially dwarfed plant.
Bonsais are regular trees or shrubs, such as juniper or cypress, that are stunted by pruning their roots and tying their branches with wire. The art originated in China around 200 C.E., and was adapted by the Japanese early in the sixth and seventh centuries.
Like barking, growling, and whining, howling is one of the few forms of verbal communication that dogs have. Its roots go back to dogs’ wolf ancestry, when wild wolves used howling to communicate over long distances. The howl swept through different pitches, which helped the sound carry over longer distances. Wolves use howling to let other pack members know their precise location if they happen to get separated.
Other members of the pack howl back in reply—an acknowledgement that the sent message has been received. Wolves also howl to discourage a rival pack from encroaching on their territory. Dogs today still display some of this behavior. If you leave your house or apartment, your dog may howl to try to reestablish contact with you. If the howling persists after you’ve left, it could be a sign of separation anxiety. And sometimes dogs howl to establish their territory.
The rays of light coming from the bottom of the pool pass from a denser medium (water) to a rarer medium (air) and are refracted (bend away from the normal).
When the rays return to the surface, they form an image of the bottom of the pool at a point, which is little above the real position.
The immune system protects the human body against germs, which are microorganisms that cause sickness and disease. There are four major types of germs— harmful bacteria (pathogens), viruses, fungi, and protozoa. This defense system begins with the skin, which stops germs from getting into your blood or tissues. If germs get into your body, for example through your nose or mouth, white blood cells called phagocytes and lymphocytes attack them. Phagocytes scout out and destroy invaders, and long-living lymphocytes remember the invaders and release chemicals called antibodies to make the body resistant, or immune, to them.
White blood cells live in the bloodstream, lymphatic system, and spleen. The lymphatic system (or lymph system, for short) is a far-reaching network that extends throughout your entire body. A clear liquid called lymph runs throughout the system, washing the body’s cells with nutrients and water and detecting and removing pathogens. Lymph is filtered through the lymph nodes, and then passes into the body’s bloodstream.
Cotton, which comes from flowering Gossypium plants, is a key vegetable fiber used for making clothes, and oil from its seeds can be used in cooking or for making soap. The cotton plant grows in 17 states that make up the U.S. “Cotton Belt”: California, Arizona, New Mexico, Texas, Oklahoma, Arkansas, Missouri, Mississippi, Alabama, Louisiana, Florida, Tennessee, North Carolina, South Carolina, Virginia, Georgia, and Kansas. In the United States, where cotton is no longer picked by hand, machines called pickers or strippers harvest the crops.
Cotton-picking machines have spindles that pick (twist) the seed cotton from the burrs that are attached to plants’ stems. Doffers— a series of circular rubber pads—then remove the seed cotton from the spindles and knock the seed cotton into a conveying system. Conventional cotton stripping machines use rollers equipped with alternating bats and brushes to knock the fluffy white bolls, which contain seeds and hairs, from the plants into a conveyor. After harvest, most of the cotton is pressed into large blocks for storage. These cotton bundles are then transported to the cotton gin, a machine that pulls out the seeds from the cotton bolls.
Absolutely. Being bitten by a plant-eating dinosaur such as Brachiosaurus, with its 52 chisel-like teeth, would certainly hurt! The bite of a Parasaurolophus, with its interlocking rows of teeth, might take off your fingers. In the Iguanodon, numerous sharp teeth were set in rows in the upper and lower jaws, and at steep angles to each other.
When the teeth were pressed together, the upper jaw was forced outward, creating a grinding motion between the teeth and its meal of crushed plant tissue.