This build party, we got an old junky microwave from a thrift store, and put any interesting thing into it, before stripping it of its transformers and turning it into a welder.
Our microwave doesn't have a turntable, but it does have an impeller, which is a piece of metal outside the oven chamber that spins and changes the magnetic resonance of the box to keep standing waves from forming.
Picturing that, or at least keeping it in mind, will give you a clearer idea of why the grapes arc inconsistently, why the lightbulbs don't stay on, etc.
Photos and videos, courtesy of Jordi Castell, posted here so you can hold your own Microwave-It night!
You can also see the original photo/images at http://evalu29.uv.es/~jordi/build_party/
Step 1: Grapes
That means that when you put them in a microwave, they resonate and the energy builds up until the juice boils and creates an ionized gas, and then arcs across the two halves.
We tried it with blueberries too, as they're almost the same size. It did work, but not quite as well.
They don't taste very good afterwards.
Here's what plasma physics on grapes looks like: