Several different factors determine how big a person will grow. The most important one is heredity, the passing of physical traits from parents to children. When you began as a single fertilized cell, your mother and father each contributed half the genes—coded chemical information—needed for you to live and grow. These genes are responsible for your physical traits, like the color of your eyes and hair, how your body will be shaped, and how tall you will become. That is why children look a lot like their parents, or even their grandparents: they have inherited family characteristics that may have been passed on for several generations. If your parents are big or tall, chances are good that you will be big or tall, too. The average height of a woman in the United States is about 5 feet, 4 inches (1.6 meters), and the average height for an American man is 5 feet, 9 inches (1.75 meters).
In spite of genetic coding, certain conditions can keep people from growing as large as their genes say they should. Bad nutrition keeps a body from reaching its maximum size. Poor health and disease do the same. That is why people who lived in generations before us, when food was sometimes scarce and health care was poor, were quite a bit smaller than we are today. Taking good care of your body, then, helps it become the best it can be.
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.
In the days of feudalism, when noblemen gathered for a meal in the castle, those of higher rank sat at the head of a T-shaped table, and the rest sat in order of diminishing importance away from them.
For such occasions a yard-long loaf of bread was baked, and the honour of making the first cut belonged to the highest-ranking person at the head table, who would then pass the bread down in order of rank, but always keeping for himself the “upper crust.”
Without the Sun, life on Earth would not exist. The planet would be a frozen dark ball, drifting in space. The Sun provides light, heat, and energy, which stirs up the atmosphere to create winds and rain. With it, plants grow, and animals and humans eat.
However, the Sun’s heat output changes over time, which affects our daily lives, the climate, and our satellite communications.
Muscles are attached to bones by tendons, the longest and strongest of which is called the Achilles tendon in your heel. This thick band of tissue attaches the muscles of the calf to the heel bone and is the key to the foot’s ability to flex.
The Achilles tendon allows you to push off of your foot when walking or running. In ancient Greek myth, the hero Achilles died from a wound to his heel, so the popular expression “Achilles heel” often refers to a physical weakness or limitation.