One of the Ten Commandments is "do not testify falsely against your neighbour"-or, put simply, do not lie. It is speculated that Christians started making the cross symbol with their fingers when lying to protect themselves against God's wrath for breaking one of the commandments. Another theory is that crossing your fingers when lying evolved from crossing your fingers for luck. In this case, you would be silently asking for luck in getting away with the lie. "Do you love me?" "Of course." "Pinkie promise?" "Fingers crossed."