Knack

n.
  1. A petty contrivance; a toy; a plaything; a knickknack.
  2. A readiness in performance; aptness at doing a specific task; skill; aptitude; facility; dexterity; -- often used with for; as, a knack for playing the guitar.
  3. Something performed, or to be done, requiring aptness and dexterity; a trick; a device.