Shortfills are under-filled bottles of e-liquid that don't contain nicotine. They have been designed to allow room for nicotine shots to be added. Add one 10ml/18mg Nic Shot to a 50ml shortfill and two to a 100ml shortfill to make 3mg e-liquid. After adding the Nic Shot make sure to shake the bottle vigorously to make sure it is evenly mixed.