Here's a quick brain-teaser for those of you who wrench on bikes: 

Ever wonder why the threads on pedals are "handed" (right-hand thread on right, left-hand thread on left)? And those English-threaded bottom brackets, the same thing.. the threads on each side go in opposite directions? It doesn't make sense if you think about a pedal bearing locking up, because it's not hard to envision the pedal "unscrewing" from the crank. Same with the bottom brackets.

What's up with that?   

Answer: "Precession". Precession is a not-so-intuitive phenomenon that assures that a properly functioning pedal will stay tight on the crank, and same for bottom brackets. If you've been around more than a few decades, some of you may remember when lug nuts were also "handed", left and right. Google it! Gives you something to talk about on those long country group rides. 

Sincerely,

Pete