A solid 'house key' is cleanly held at the magician's fingertips.  Then, incredibly the 'Bit' of the Key is dragged down the shaft, from one end to the other. It looks incredible and extremely magical. What's more, the key is given out for full examination (no switching) with the bit solid and unable to move from the inner end of the key shaft.

 

A very effective trick, that's both easy to perform and extremely baffling.