Hammering a nail into a wall always carries the danger that you'll miss and crush your fingers or thumb. This is especially likely when you first start hammering. Avoiding this mini-disaster is surprisingly easy. Use a piece of thin cardboard to hold the nail in place -- poke the nail through the cardboard and hold the cardboard instead of the nail while you hammer away. Once the nail starts to go into the wall, remove the cardboard and finish hammering the nail fully into the wall.