People often tell this joke about Einstein but I first heard it from Issac Asimov.

A young man has an appointment with Johann Carl Friedrich Gauss. The great man is not in when he first enters his office but he notices a horseshoe behind the desk, hung up in the appropriate manor so all the luck will not fall out. When Gauss arrives the young man asks what it is doing there and is he superstitious? " Of course not " Gauss replies " but I am reliably informed be people who know of such things. That it works whether you believe in it or not ".

Many years ago I used to busk for a living. Amongst other things it would entail trimming my nails so as to play guitar. Then I heard a superstition that said it was bad luck to cut your nails on a Friday. Every time I did this I made a lot less money, so I would trim my nails on another day instead. Yes I do know this was probably psychosomatic but made more money so...