Well, it all comes down to Flann O'Brien and the theory of melocular interchange - at least, where it applies to bicycles.
The short answer is, don't stand still in one place for too long, lest you become assimilated(sp?).
(cf: The Third Policeman - one of the funniest books ever written in the English language)
Failing that; just try to get a bus in London. Now THERE"S unexplained for you...