Subject: Why don't shrinky dinks shrink in the microwave oven?

For my science project, I made Shrinky Dinks from #6 recycled plastic 
(polystyrene).  To make them, you sand the plastic, color it in, and put it in a 
toaster oven for a few seconds.  It gets smaller and thicker.  In the
oven, the plastic shrinks.  In the microwave, it doesn't.  It just gets warped a 
little after a long time in the microwave.  I know why heat makes the
plastic shrink.  Why doesn't this work in the microwave?

Added after requesting more information:
Thank you for your reply.  I'm not sure how to resubmit this, but here is the
answer to your question.  The shrinky dink got warm in the microwave, but
not hot.  I didn't even need hot pads to touch it.  I put the shrinky dink
in the toaster oven for 30 seconds.  I put one in an old microwave for about
8 minutes on high power.  I hope this helps you understand.  Thank you.
Devin.

Re: Why don't shrinky dinks shrink in the microwave oven?