What's with this thing?

bi*fur*cate--to divide into two branches or parts

What is going on when you get that bifurcated stream? What stream, you're asking. It's that stream that occurs when you have to go to the bathroom and you stand there and your stream bifurcates, with part flowing into the toilet and the other part landing:

a) all over the floor
b) all over the toilet
c) (and this one is the best) all over your pants

What makes the urine stream split? Some sort of blockage? It also seems to happen at the most inopportune times (as if there is a best time to urinate all over your pants).

"Kevin are you ready to go disc golfing?"
"Yeah, just a second"
(60 seconds later)
"Woah, what happened?"
"Um, the water splashed out of the sink really weird."
"Loser."

Or

"Kevin are you ready for church yet?"
"Yeah, just a second, mom.......Dang it!"
(60 seconds later)
"Ummm"
"Yeah, I was washing my hands and put my finger up against the water faucet part and it built up the pressure really hard and shot water in a perfect stream over the sink and all over the side of my pants here as it is kind of obvious to see. What luck, huh?"
"Loser."

Does anybody have an answer?

Stay high and dry out there, kids.