Capillaries are small thin-walled vessels that permit the exchange of oxygen, carbon dioxide, nutrients, and waste between blood and the body's cells. This process of exchange is called diffusion.

Unlike curved mirrors that operate on the principle of reflection, lenses utilize refraction. A convex lens is thicker in the middle than on the edges and converges light while a concave lens is thicker on the edges than in the middle and diffuses light. A common use for curved lenses is in eye glasses where a convex lens is used to correct farsightedness and a concave lens is used to correct nearsightedness.

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.

An element in a solid state has atoms or molecules that are constricted and do not move freely. Solids maintain a constant volume and shape and exist at a lower temperature than liquids or gases.

The four planets closest to the Sun (Mercury, Venus, Earth, and Mars) are called terrestrial (Earth-like) planets because, like the Earth, they're solid with inner metal cores covered by rocky surfaces.