用listen找隐形生物——
A creature can use hearing to find an invisible creature. A character can make a Listen check for this purpose as a free action each round. A Listen check result at least equal to the invisible creature’s Move Silently check result reveals its presence. (A creature with no ranks in Move Silently makes a Move Silently check as a Dexterity check to which an armor check penalty applies.) A successful check lets a character hear an invisible creature “over there somewhere.” It’s practically impossible to pinpoint the location of an invisible creature. A Listen check that beats the DC by 20 pinpoints the invisible creature’s location.