Soul Music by Terry Pratchett
'But... look, there's no wall here, there's just...'
Albert wrenched open a drawer.
'Observe,' he said sharply, 'Hammer, right? Nail, right? Watch.'
He hammered the nail into the air about five feet up at the edge of the tiled area. It hung there.
'Wall,' said Albert.
Susan reached out gingerly and touched the nail. It had a sticky feel, a little like static electricity.