Part of the peripheral nervous system, the autonomic nervous system regulates involuntary activity in the heart, stomach, and intestines.

The liver produces bile which emulsifies (separates) fat.

Small quantities of certain minerals like iron, calcium, magnesium, and salt are important for nutrition and health.

The narrowest classification of life, species, contains organisms that are so similar that they can only reproduce with others of the same species.

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.