Balloon Propelled Toy Car

I have to begin by giving my 4 year old's pre-school provider props for this one.  The other week my 4 year old's pre-school provider helped each of the students build balloon propelled toy cars, as seen below.

How does it work?  It starts with Newton's 3rd Law of Motion which states that for every force there is an equal and opposite reaction force.  The balloon is filled with air and held tight at the end.  When the end of the balloon is released, the greater outside pressure force collapses the balloon and pushes the air out.  The air from the balloon pushes against the air outside the balloon, which in turn pushes right back, propelling the car forward. 

Very cool!  Build your own and try out larger balloons.  What works best?  What type of balloon gives the greatest car speeds?  There are many things one can test out here.