As an online discussion grows longer, the probability it being reduced to dick jokes approaches 1.
c.f Godwin's Law
Note: invoking Cathcart's Law massively increases the probability of dick jokes, as it contains the fragment "grows longer".
(following a lengthy conversation about GPS)
Steve: My GPS has a 3" screen, and it's perfectly adequate for my needs.
Tim: Yeah, but how long do you want to be staring at a 3" thing?
Tim: Cathcart's Law!