When something is burning, it is said to be on fire. Flames can be different colours depending on what is burning and how hot it is. Fire is very hot and bright, and people light fires in their houses to keep warm. When a fire is out of control it can be extremely dangerous, as the photo of a forest fire shows. When something burns it gives off gas and particles as smoke. It gives off heat because burning is an exothermic reaction, which means it creates heat as it happens. Some things like boiling water are the opposite, and need heat to happen.