Also, and confusingly, if you cross your fingers behind your back while you are telling a lie to someone, it is supposed to "neutralize" the lie somehow.
Child a: "You've eaten the last piece of pie! You told me I could have it!"
Child b: "Yeah, but I had my fingers crossed."
This is a custom of childhood, but I've seen it used by adults on television comedies, etc. For instance, this
book cover art: