Uh. this might be a stupid question
So anyway, I took it upon myself to change my wheel bearings... I'm trying to put it back together now (The steering knuckle)... how the hell do I stop the ball joint from twisting while i'm trying to put the nut on?

 

After everything else is connected to the control arm and knuckle (eg strut, stabilizer bar, ect.), I just place a floor jack under the control arm (directly under the balljoint) Jacking it up until the spring is compressed a little will provide enough friction where the ball joint seats in the knuckle to allow you to tighten the nut.