Deoxyribonucleic acid (DNA) is the molecule that contains genetic information. DNA is encoded through a combination of nucleotides that bind together in a specific double helix pattern.

The Earth is approximately 4.6 billion years old and its history is divided into time periods based on the events that took place and the forms of life that were dominant during those periods. The largest graduation of time is the eon and each eon is subdivided into eras, eras into periods, periods into epochs, and epochs into ages.

Because different materials have different refractive indices, light changes speed when passing from one material to another. This causes the light to bend (refraction) at an angle that depends on the change in refractive index between the materials. The greater the difference, the higher the angle of refraction.

Part of the peripheral nervous system, the somatic nervous system is made up of nerve fibers that send sensory information to the central nervous system and control voluntary actions.

Clouds are categorized based on their shape, size, and altitude. Stratus clouds are low-altitude clouds characterized by horizontal layering with a broad flat base. When stratus clouds occur on the ground the result is fog.