While flipping through the channels the other night I came across bull riding. In watching a little of it I had some questions as to how the sport works.

Most important question is who picks the bulls?

Do the riders pick their own bull? Or do they bring their own with them? Maybe it is some jury picking type process whereby all of the bulls are led into the ring and the riders rank their favorite.  The reason this is important is because it doesn't seem fair. I mean what if one rider stays longer on his bull because it was more tame than another? Or is there some minimum level or "pissed offedness" that a bull must possess in order to qualify?

I'm trying to expand my horizons.