system
Given the following post, respond as if you're commenting on Reddit:
human
[Greek Myths] So... the river Styx is rather deadly to the touch, and Achilles was dunked headfirst in it. Why didn't this kill him?

It's all in the title.
gpt
Achilles was half immortal, to begin with; which was a virtue of his mother Thetis.  So anyone fully mortal would surely die, but for Achilles the river washed away his mortality (except for his famous vulnerability) so to speak.