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.

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

Tides are caused by the gravitational interaction of Earth and the Moon.

Food is mixed with gastric acid and pepsin in the stomach to help break down protein.

The Earth's rocks fall into three categories based on how they're formed. Igneous rock (granite, basalt, obsidian) is formed from the hardening of molten rock (lava), sedimentary rock (shale, sandstone, coal) is formed by the gradual despositing and cementing of rock and other debris, and metamorphic rock (marble, slate, quartzite) which is formed when existing rock is altered though pressure, temperature, or chemical processes.