Veins carry blood back to the heart from the body. While arteries are thick-walled because they carry oxygenated blood at high pressure, veins are comparatively thin-walled as they carry low-pressure deoxygenated blood. Like the heart, veins contain valves to prevent blood backflow.

The water then accumulates as runoff and eventually returns to bodies of water or is absorbed into the Earth (infiltration) and becomes part of the water table, an underground resevoir of fresh water.

Heredity is the passing on of physical or mental characteristics genetically from one generation to another. Heredity is made possible via large strings of chromosomes which carry information encoded in genes.

The cerebrum is the major part of the brain and is responsible for the main senses (thinking, hearing, seeing).

A comet is a loose collection of ice, dust, and small rocky particles that, in contrast to an asteroid, has an extended atmosphere surrounding the center. When passing close to the Sun, this atmosphere warms and begins to release gases forming a visible coma or tail.