Torrent of Lava deals X damage to each creature without flying.
As long as Torrent of Lava is on the stack, each creature has ": Prevent the next 1 damage that would be dealt to this creature by Torrent of Lava this turn."
10/4/2004 If X is zero, it deals no damage (not -1 damage) to non-flying creatures it taps.
8/1/2008 It grants the activated ability to creatures while on the stack. This allows the ability to be activated and resolved before the Torrent itself resolves.