All probabilities are fiendishly tricky

Randall Munroe says this today after posting a probability puzzle:

I love puzzles which are simple to state but have a fiendishly tricky or counterintuitive answer.

This must be why he can be a NASA roboticist and successful math/science webcomic author: for me, every probability puzzle is fiendishly tricky, and I usually love math puzzles. Probability is one subject that has always evaded my grasp - every correct solution seems completely illogical.

I won't even bother trying to figure out his posted puzzle:

Sue and Bob take turns rolling a 6-sided die. Once either person rolls a 6, the game is over. Sue rolls first. If she doesn’t roll a 6, Bob rolls the die; if he doesn’t roll a 6, Sue rolls again. They continue taking turns until one of them rolls a 6.

Bob rolls a 6 before Sue.

What is the probability Bob rolled the 6 on his second turn?

Link: http://blag.xkcd.com/2009/02/11/a-math-problem-2/