i believe i've read about this somewhere...
there's two "rivers" that run through the penis. they switch from "piss" mode to "cum" mode. when you're hard, the "cum river" is enlarged, or in the gateway, or some shit like that, and when you aint hard, the "piss river" is in the gateway.
and yeah, it's a bitch..... i can't piss when im at school cause i always got a hard-on for the fine-looking females, so i gotta wait for like 2-3- minutes before i can piss, and people comin in and out stare at me like im jerkin it in silent mode.