How Do Humans Know They Have To Pee?

do you ever think of geysers and waterfalls? Hundreds of thousands of gallons of water, tumbling and flowing, spilling and falling

Yes; and has anyone else noticed how totally sadistic movie directors will put completely gratuitous water scenes in near the end of the movie? I’m not talking about Titanic, you know there’s gonna be water in that one, but does somebody really have to run a bath (or for that matter be drowned in a bathtub), or stand next to a fountain, or anything that involves running water in the last fifteen minutes of the movie? Sheesh.

A friend and I missed a portion of the movie Cast Away because of this!
Had…to…get…to…restroom…and it was a bit of a hike!

The joker: Y’know how it feels when you have to pee?
The patsy: Yeah.
The joker: Here, feel me and see if I have to pee.

The patsy who has heard the joke already: (feeling the joker’s forehead) No, you don’t need to pee. You’re nearly fulla shit, though.

James Lileks described the feeling this way:
“…as though a hot and highly pressurized basketball has materialized in your lower abdomen.”

Is it just me, or does this sound like the question posed by a cruel experiment from a Harry Turtledove novel?