Potassium is a soft metallic element that is essential to all living things. It is the eighth most abundant metal, making up 2.1% of the earth's crust by weight, and next to iridium, it is the lightest known metal. It can be easily cut with a knife because of it softness, and its shiny silver color can be seen when a new surface is exposed. To prevent it from reacting with the oxygen and water in the air, samples of metallic potassium are usually stored submerged in a mineral oil.