why do we cross our fingers?

I asked a Chinese student to present two idioms to my speaking class, and he chose "keep your fingers crossed" and "a little bird told me". First he explained their meanings, good luck and a secret source.

But then he explained the history of these common expressions. People used to ward off vampires by making the sign on the cross, crossing their fingers. (Take that, Edward Cullen!)

"A little bird told me" was even more bizarre. It was forbidden to say anything bad or have evil thoughts about the king, because the thoughts could drift away out the window, like a little bird. So a little bird signifies your secret thoughts. Isn't that wild? This reminds me of that sign that says to watch your thoughts because they may become words, which become action or reality.