From our good friends at fortune

An American scientist once visited the offices of the great Nobel prize
winning physicist, Niels Bohr, in Copenhagen. He was amazed to find that
over Bohr’s desk was a horseshoe, securely nailed to the wall, with the
open end up in the approved manner (so it would catch the good luck and not
let it spill out). The American said with a nervous laugh,
“Surely you don’t believe the horseshoe will bring you good luck,
do you, Professor Bohr? After all, as a scientist –”
Bohr chuckled.
“I believe no such thing, my good friend. Not at all. I am
scarcely likely to believe in such foolish nonsense. However, I am told
that a horseshoe will bring you good luck whether you believe in it or not.”