Fire is the rapid oxidation of a material in the exothermic chemical process of combustion, releasing heat, light, and various reaction products. Slower oxidative processes like rusting or digestion are not included by this definition.

The flame is the visible portion of the fire. If hot enough, the gases may become ionized to produce plasma. Depending on the substances alight, and any impurities outside, the color of the flame and the fire's intensity will be different.

Fire in its most common form can result in conflagration, which has the potential to cause physical damage through burning. Fire is an important process that affects ecological systems across the globe. The positive effects of fire include stimulating growth and maintaining various ecological systems. Fire has been used by humans for cooking, generating heat, signaling, and propulsion purposes. The negative effects of fire include water contamination, soil erosion, atmospheric pollution and hazard to human and animal life.

In fiction and folklore, a doppelgänger (German "double walker") (pronounced [ˈdɔpəlˌɡɛŋɐ] ( listen)) is a paranormal double of a living person, typically representing evil or misfortune. In modern vernacular it is any double or look-alike of a person.

It also describes the sensation of having glimpsed oneself in peripheral vision, in a position where there is no chance that it could have been a reflection. Doppelgängers often are perceived as a sinister form of bilocation and are regarded by some to be harbingers of bad luck. In some traditions, a doppelgänger seen by a person's friends or relatives portends illness or danger, while seeing one's own doppelgänger is an omen of death.

In Norse mythology, a vardøger is a ghostly double who precedes a living person and is seen performing their actions in advance. In Finnish mythology, this is called having an etiäinen, i.e., "a firstcomer". In Ancient Egyptian mythology, a ka was a tangible "spirit double" having the same memories and feeling as the original person. In one Egyptian myth titled "The Greek Princess," an Egyptian view of the Trojan War, a ka of Helen was used to mislead Paris of Troy, helping to stop the war. In some myths, the doppelgänger is a version of the Ankou, a personification of death.