The ancient Greeks believed that all matter was made up of four elements: water, fire, air, and earth. Water could be described by its chemical formula of H2O and role as a universal solvent. Fire requires oxygen, fuel, and heat to exist but can be dangerous if uncontrolled. Air is invisible but takes up space and has mass, enabling the exchange of gases between plants and animals. Earth's crust contains oxygen and silicon and its layers provide metals used to make tools and buildings.