well I don't think a cd contains enough water to show any effect in the microwave, unless of course there are lots of files on it like water.jpg, ocean.bmp, waves.gif(animated of course)...

OK, enough chiding. There IS of course a way to use the CD.
Go to the kitchen (forgot, you are there already, or where do you have the microwave?), get yourself some aluminium foil and cut it so it EXACTLY fits the CD. The tricky thing is, dont get it crumpled. You then glue it to the CD (Honey or any sugar-based softdrink works well as adhesive, as long as it's no "LIGHT".

The beauty is, you can even have double-sided media this way!