And nuke it.
Things to do in a microwave #2: Create a plasma
It just so happens that grapes are about the size of the wavelength of microwaves, which is important. And grapes also have sugars, which make them into dielectrics. (There are other fun things you can do with grapes because of this). Both of those together make the coupled grape halves into a dielectric dipole antenna, which is just a fancy way of saying that the microwaves that hit one side of the cut grape will pass to the other side, in a very concentrated way. The result is that there is a huge voltage generated between the two sides of the cut grape. That voltage causes electricity to jump from one grape half to the other (”arcing”). This is what happens when you rub your socks on the carpet and touch the doorknob — that spark is electricity jumping from your hand to the doorknob. The difference in this case is that there is a HUGE voltage generated (3000 volts by one website), and that is enough to ignite the steam from the grapes into a plasma state (a glowing ionized gas, where the electrons have been ripped from the gas molecules by the high temperatures). You can capture this plasma in a glass, as in the video above (wow!)
And, of course, this is preceded by Things to do in a microwave #1: Find your microwave hot spots
In addition to the two methods Stephanie lists, you can use marshmallows or chocolate chips, and look for where the melting starts. And then you can eat the experiment. (Stephanie mentions marshmallows; I missed it)
Update: Not done yet! Things to do in a microwave #3: Ivory Soap Monster
Things to do in a microwave #3: Microwave a CD
#3? Should be #4. (I’ve brought the whole “Five is right out!” counting thing to Stephanie’s attention) There’s an image that shows some mini Lichtenberg figures, i.e. the little tree-like patterns the electrons make.