Traditionally, resources have been categorised as either renewable or nonrenewable. However, natural resources fall into three types: renewable, non-renewable, and continuous. These classifications are evolving as we put more strain on the earth's natural capital, and certain resources' definitions are shifting. In a nutshell, continuous resources are resources that are always available in a steady supply, such as solar, wind, and tidal energy. Non-renewable resources are only available in a specific location and in finite quantities since the rate at which the resources regenerate is exceedingly slow on the human timescale. Renewable resources recover spontaneously as long as their ability to do so is not irrevocably harmed. However, not all resources are purely categorised according to this simple scheme. Some stock resources, such as metal ores, are more difficult to categorise. They are typically regarded as non-renewable stock resources, however they can be reused. They are able to be extracted once as stocks, but the refined metals can be recycled as scrap. Even though their parent ores are not, the resource products are.