Subject: Please explain how acetone causes the molecular breakdown of styrofoam?

My daughter wants to do an experiment for her 4th grade class that we saw 
performed.  The scientist squirted pure acetone on a styrofoam plate and 
then a styrofoam cup which caused them each to stretch, breakdown, etc.  We 
have all of the items to perform the same experiment but we can't find ANY 
basic scientific explanation that would be appropriate for a 4th grade 
class. Please explain basic properties of acetone, styrofoam and they react 
together.  Thank you very much.

Re: Please explain how acetone causes the molecular breakdown of styrofoam?