Plastic is not a new material. It has been around for a long time. What has changed over time is the availability of plastic. Plastic has become a popular choice for many people because it is easy to use, easy to clean and very durable. Many people also like the idea of recycling plastic. However, there is one thing that is not so easy to change – the melting point of a substance. There is a temperature at which a substance melts and becomes liquid. This is known as the melting point of the substance. Some materials have a low melting point, meaning that they melt quickly. Others have a higher melting point, meaning that they melt at a slower rate. One material with a high melting point is salt. Salt does not melt at temperatures that are too high. In fact, it has a melting point that is so high that it is the coldest natural substance. It is known as the thermometer of cold.