What do you tell people when you have to take a piss?

[The Ghost of RubangB]

Dammit, I was just about to say "open your mouth," but I was too late.

I usually say, "I gotta urinate." None of the euphemisms are good enough for me.

Or I just start quoting RE and say "Whoa whoa, take a look at THIS! It's a weapon, and it's very powerful, especially against living things!" and I run to the nearest urination zone.