Like the respiratory system, the circulatory system serves to transport oxygen throughout the body while removing carbon dioxide. In addition, the circulatory system transports nutrients from the digestive system.

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.

An element is matter than cannot be separated into different types of matter by ordinary chemical methods.

The columns of the Periodic Table are called groups and all elements in a group have the same number of electrons in their outer electron shell. The group that an element occupies generally determines its chemical properties as the number of outer shell electrons establishes the way it reacts with other elements to form molecules. So, because each element has the same number of electrons in its outer shell, each has similar reactivity.

A prefix is added to the base units of the metric system to indicate variations in size. Each prefix specifies a value relative to the base unit in a multiple of 10. Common prefixes are: