How exactly do screws work?

This was a question on Quora. For some reason it caused me to go into full auto blather mode. Here is my answer:

Since I have been working on a house recently, when you ask about screws I think of wood screws. Wood screws are not made of wood, not to say that you couldn’t make a screw out of wood (and it has been done), but the ones I have been using are made of steel and are used to hold two pieces of wood together. Wood screws are all about friction and relative strength. Wood screws have a single sharp edge that spirals around the shaft of the screw from the tip to somewhere short of the head. The hard, sharp edge of the steel spiral cuts into the softer wood and creates a groove that the rest of the spiral follows as you turn the screw. Following this self cutting groove draws the screw into the wood.  You need a bit of a hole in the wood to get the screw started.
Traditionally you would drill two holes, one into each of the two pieces of wood you are trying to join. The outer piece, the one you are trying to attach to the inner piece, or ‘base’, gets a clearance hole, that is, a hole large enough for the screw to slip inside, but small enough to prevent the head from entering. The base piece gets a pilot hole that is roughly the same diameter of the portion of the screw that holds the spiral, measured without the spiral. In hardwoods, like you would use for making furniture the pilot hole needs to be a little larger. For soft woods, the hole would be a little smaller.
Modern wood screws used in construction with softwood only need a bit of a dent in the wood to get started. That can be provided by simple pushing the tip of the screw into the wood. Once the spiral catches hold of the wood, it forces its way into the wood as it is turned.
That’s how you get the screw into the hole. What holds it there? Friction. A screw with a well made thread that fits the hole with just a bit of clearance can be screwed in with little or no force. You can demonstrate this with new machine screws easily. Put a nut on a long bolt and give it a spin and it will screw on or off the bolt due to inertia. You can also see this with the lug nuts used to hold a wheel on a car. Examples with wood are less common.
When a screw is tightened, the head is forced against the piece being held, and the spiral ridge is forced against the upper side of the groove, squeezing the wood between them and stretching the screw. This longitudinal force increases the friction between the piece the screw making it difficult to unscrew. Screws still come undone, mostly in metal machines that vibrate. This is why critical bolts in airplanes are safety wired to prevent them from unscrewing while the plane is in flight.