How friction keeps nails from falling out of the wall?

Imagine a small room, maybe 3ft x 3ft, with six fully grown adults packed inside like sardines. Now image you were somehow squeezed into that room. Do you think you would be able to move around freely? What's stopping you from dropping to the floor and doing 10 push-ups?

When a nail is driven into a block of wood it doesn't remove bits of sawdust like a screw does. In fact the wood that should have been removed to make way, is violently compressed and desperately trying to get back into its regular station of life - occupying the space said nail has now so churlishly taken.

This extreme pressure creates the friction needed to ensure the nail is a permanent addition to our block of wood.

The lifelong, unwavering pressure on our nail will never tire, never relent, keeping it firmly in place until elements and old age give way to rotted wood.

Now go hang a picture.