The Periodic Table of the Elements categorizes elements primarily by the number of protons in their nucleus (atomic number) and secondarily by the characteristics they exhibit.

A cold front is a warm-cold air boundary with the colder air replacing the warmer. As a cold front moves into an area, the heavier cool air pushes under the lighter warm air that it is replacing. The warm air becomes cooler as it rises and, if the rising air is humid enough, the water vapor it contains will condense into clouds and precipitation may fall.

An electron is a subatomic particle that orbits the nucleus of an atom. It carries a negative electric charge. Generally, an atom has the same number of negative electrons orbiting the nucleus as it does positive protons inside.

The trachea branches into the left and right bronchi which each lead to a lung where the bronchi subdivide into smaller tubes called bronchioles. Each bronchiole ends in a small sac called an alveolus which allows oxygen from the air to enter the bloodstream via tiny blood vessels called capillaries.

The water (hydrologic) cycle describes the movement of water from Earth through the atmosphere and back to Earth. The cycle starts when water evaporates into a gas from bodies of water like rivers, lakes and oceans or transpirates from the leaves of plants.