Q:    In some movies scenes involving explosives actors are asked to keep their mouth open and warned that if they don't their eardrums could implode.  Why would a blast from explosives require the actors to keep their mouths open?

A:    When the explosion goes off there is a pressure wave caused by the rapid burring of materials and thus the rapid change in volume of the explosive.  If the actor's mouth is closed, air at the pre-explosion pressure is trapped in the actor's sinus area.  The shock wave causes an area of high pressure to form around the actor.  When the actor's mouth is closed, the difference of pressure between the sinus area and the outside air can cause the ear drums to burst inward.  However, if the actor's mouth is open, the pressure wave will be the same both inside and outside the actor's mouth and the actor should be safe.  This same effect happens (although with less severity) when you go diving or when you go from high to low altitudes.