In "There Will Come Soft Rains," what are the "green snakes?"
The central character of "There Will Come Soft Rains" is an automated house that serves its inhabitants and oversees safety and security. Near the end of the story, a fire breaks out and the house tries to fight it, but is low on resources and eventually fails. Throughout the story, Ray Bradbury personalizes the house and its robotic workers, humanizing them as replacements for the long-dead owners. The "Green Snakes" are streams of fire-fighting chemical:
The house gave ground as the fire in ten billion angry sparks moved with flaming ease from room to room and then up the stairs.
From attic trapdoors, blind robot faces peered down with faucet mouths gushing green chemical.
The fire backed off, as even an elephant must at the sight of a dead snake. Now there were twenty snakes whipping over the floor, killing the fire with a clear cold venom of green froth.
(Bradbury, "There Will Come Soft Rains," jerrywbrown.com)
The fire and house are both humanized, the fire angry and the house determined, and the "blind robot faces" part of the house's automatic fire-fighting equipment. As the green chemical sprays, the fire retreats, but finds new hold in the attic. By comparing the green chemical to "snakes" and the fire to "an elephant," Bradbury shows both how emotional reaction can be given to non-biological things, and how even something small can significantly affect something large; in this case, the raging fire is almost defeated by the small "green snakes," but wins out in the end through sheer brute force.