Here’s what you do — slice a grape in half, but keep the halves connected by a little “hinge” of grape skin.  Some suggest drying off the grape halves a little.  Some suggest using a green grape in particular, and some say to cut it in quarters. Put the two halves next to each other, face up, in the microwave.  It’s best to place it slightly off-center, as microwaves have hot-spots and “nodes” and the center is a “node” of radiation.  Press go.  Here’s what happens:

So, what’s going on?

Microwaves are just one form of radiation — like x-rays or visible light — of a particular frequency.  The reason they chose microwave frequencies for ovens is that this frequency is more readily absorbed by water.  When microwaves hit water molecules, it makes them vibrate.  That vibration gets turned into heat.  Grapes are watery, so any grape will heat up when put in a microwave.

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!)

So, in other words, the grapes concentrate the electric fields in the region between them, strongly enough to ignite the air and make a plasma.

Here is a nice detailed write up about this (which also includes some safety cautions if you’re concerned about ruining your microwave) and another less nerdy explanation that I think might have some inaccuracies in its attempt to put this into everyday language.

And in slow motion:

[youtube=http://www.youtube.com/watch?v=vb6WCkC0K1I]

And a more experimental approach:

[youtube=http://www.youtube.com/watch?v=qvSxVBalhFM]

And a detailed explanation:

[youtube=http://www.youtube.com/watch?v=Ejs5twFVnfU]

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