What a surprisingly polite site! Nearly everyone says "pee". It's "pee" left, right and centre! Even the site's name is called peefans. My point? Well, "pee" is the polite word, one that will not cause offence in public - it's like the casual version of "urinate", that children are allowed to use. "Piss", however, is the rude word, never for public use in polite company. Seeing that this is the ideal place for it, I urge you to try saying and writing "piss", not just because it's rude but because it's onomatopoeic. Go on: I urge you to try it. You won't regret it - it's so much more fun.