Explosive chemical reactions are what send spacecraft into space. A rocket burns fuel to produce a jet of hot, expanding gas. What fuel is used varies, but whatever the mixture, it causes the explosive chemical reaction.
Because a rocket needs thrust to escape Earth’s gravity, the explosive chemical reaction takes place in a confined chamber and releases gases into a cone-shaped nozzle out the back end of the rocket. The cone shape accelerates the gases and they blast out of the engine at up to 9,941 miles (15,998 kilometers) per hour.
Once seeds are fully developed, they need a good place to grow. If they just fell to the ground beneath their parent plant, they would struggle, competing against each other for sunlight, water, and minerals. Most seeds need to travel—by wind, water, or with the help of insects and other animals—to better places to germinate, or start to grow into new plants. Some seeds, like those from conifer and maple trees, have wings attached. Others, like those of dandelions, have parachutes made of tiny hairs. Both features allow the seeds to be carried great distances by the wind, and they sometimes land in spots that are good for germination. Water carries other seeds to good growing places; the hard, watertight shell of a coconut, for instance, allows it to travel many miles at sea before finding a beach where conditions are suitable for growth.
Seeds sometimes have to wait a long time before they find good places to grow, places where the sun, moisture, and temperature are right. Most seeds are designed for the wait, protected by a hard outer pod (except those of conifers). Some seeds wait years to germinate, and some just never do. But inside each seed pod is a baby plant, or embryo, and endosperm, a supply of starchy food that will be used for early growth if germination takes place. Then a tiny root will reach down into the soil, and a tiny green shoot will reach up, toward the light.
The word “planet” comes from the Greek word for “wanderer.” Ancient astronomers defined planets as objects that moved in the night sky against a background of fixed stars. Today, astronomers define a planet as an object in orbit around the Sun that is large enough (massive enough) to have its self-gravity pull itself into a round, or nearly round, shape.
In addition, a planet orbits in a clear path around the Sun—there are no other bodies in its path that it must “sweep up” (or clear) as it moves around the Sun.
Husbands and wives divorce when they can no longer live happily together. It is usually a sad thing, because when people marry they expect to be with their partner for the rest of their lives. But over the course of a marriage things happen, people can change, and the happiness that the couple was so sure of in the beginning sometimes disappears. When couples with children get divorced it is even more unfortunate because more people are affected. Many children feel bad when their parents divorce because their family will not be the same. After a divorce, they generally do not see one of their parents as much as they did before.
Still, just because the feelings between a mother and a father change doesn’t mean that their love for their children changes in any way. It’s important to remember that divorce is something that happens between a husband and wife—it has nothing to do with the kids. Many children feel that if they adjust their behavior somehow their parents will want to stay together, but divorces are not caused by anything children do.
Fixed groups of stars that seem to form a particular shape, such as that of a person, animal, or object, are called constellations. Astronomers have identified 88 constellations and many of them represent characters from Greek and Roman mythology.
For example, the name Hydra, the largest constellation, comes from the water snake monster killed by Hercules in ancient mythology. Some of the constellation names are in Latin; for example, Cygnus means Swan and Scorpius means Scorpion.
Maybe. Warts are skin infections caused by a common virus called HPV (human papillomavirus). They are perfectly “normal,” in that health researchers estimate that three out of four people will develop a wart some time in their lives, usually on their hands or feet. It can take months or years for a wart to disappear on its own. To speed up the process, some skin doctors recommend wrapping the wart in duct tape until it disappears.
The duct tape removes dead skin from the wart, thereby gradually killing off the wart virus that lives in the skin. It may also trigger the body’s immune system to attack the wart virus. Otherwise the wart can be removed by a doctor with a laser or liquid nitrogen, a substance that freezes the skin, killing the cells.
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.
Along with space exploration came new expressions that are now everyday language. Astronauts said “affirmative” for yes, “check” to confirm a completed task, and “copy” to indicate that an instruction was understood. “Glitch,” an unexplained computer malfunction, was first used to describe the Mercury space capsule’s frustrating tendency to signal an emergency when none existed.
Because animals are unpredictable and because schools have rules, it is never okay to bring animals out in public without the permission of both a parent and teacher. Even if you have permission, these creatures—such as snakes, rabbits, hamsters, gerbils, lizards, frogs, and other small critters—should be contained in their habitats and handled very carefully. Sometimes in public animals are outside of their natural habitats because they play a role in helping humans.
Guide dogs (also called seeing-eye dogs) help the blind or visually impaired move around cities and neighborhoods. These dogs are trained to behave properly on public transportation, in restaurants, in stores, and in any other place their masters go. Other times people carry their dogs and cats in pet carriers to the vet, to the park, or on shopping trips. However, they must have the store’s permission to enter, since many businesses (like restaurants and supermarkets) do not allow pets or require them to be contained and/or on leashes.
A disability can be the result of a disease, an accident, or of genetics, which means that it is a condition that a person is born with.
A lot of times disabled people can learn new ways to do things or use special machines or specially trained animals to help them work around their disability.