In the Twelve Labors of Hercules, Hercules used fire to stop the Lernaean Hydra’s severed necks from regrowing.
The Hydra’s most distinctive power was regeneration: cutting off one head could cause additional heads to appear. Hercules therefore needed a method that prevented the wounds from producing new heads. In the best-known version, Iolaus used burning brands to cauterize each neck after Hercules cut off a head.
This episode explains why Iolaus is an important figure in the Hydra story even though Hercules performed the labor. Eurystheus later claimed that outside assistance made the feat invalid.
The Hydra’s final immortal head was treated separately. Hercules buried it under a huge rock and used its poisonous blood on arrows. Thus fire stopped regeneration, while the monster’s venom became a later weapon.