How nails and screws are held in wood by friction?

How? Well, a nail is forced between the fibres of the wood - those fibres now have to stretch in order to go around the nail. This puts them under considerable tension - and that means that they press inwards against the sides of the nail. read more

The tip of the screw is pressed into the wood and as you turn it, the spiraling threads of the screw wedge themselves in between the grain fibers and pull the screw down into the wood. In short: nails are held in by pressure/friction of the wood fibers. Screws are held in by their threads being wedged between the fibers. read more