I once saw this demonstrated live on a kids' TV show. The guy was basically a crazy scientist with very little idea what kids could actually understand.

His trick must not have been vetted before he went on, so as he started putting screws in the microwave and trying to explain things, the poor hosts were getting more and more flustered and concerned. It was hilarious.

ADDED IN EDIT: My apologies to efnx - when I saw that the the submitted link referenced another site, I thought it was just referencing it. The comments below show that the referenced site was actually doing something different.

I'll leave this here as a reference, but I was wrong.

mea culpa

========

Here's the actual link:

http://gaurabc.com/visualizing_microwaves_in_a_microwave_ove...

From the guidelines at http://ycombinator.com/newsguidelines.html:

  In Submissions 
  ...
  Please submit the original source. If a blog post
  reports on something they found on another site,
  submit the latter.

One uses neon bulbs embedded in a clear sheet, the other uses marshmallows. This link provides a substantially easier-to-replicate version, and is therefore valuable.

You're right, I've added an apology.

Huh, I thought this would be original enough as I sited the original link in the post itself, which has more of a 'more great things to do with a microwave' theme than a 'check out this other post' theme (which is to say that this post is almost completely different). Sorry.

You're right, I was wrong, and I've added an apology, repeated here.

My apologies.

Ahh - thanks! Much appreciated. I was worried I broke the rules.

You can do this with a plate of chocolate chips too, if no marshmallows are handy.