A screw is a sharp piece of threaded metal similar to a nail. Unlike a nail, a screw has spiral grooves down its spike. A screw is pressed down against wood and turned with a screwdriver. Turning the screw to the right makes it go into the wood. The grooves cut into the wood. This prevents the screw from falling out, as a nail might. Turning the screw to the left pulls it out of the wood. 