Plasma is what's responsible for St Elmo's Fire. It's the solar wind that whips through our planetary system. It's part of the ionosphere. It's part of the sun. And you can create it in your microwave, with a grape (provided you have no further use for either the microwave or the grape.)
Plasma is what happens when electrons are stripped away from the nucleus they orbit, and stay away from it, even though they're still stuck in the same medium. This can happen under conditions of high difference in electric potential, like between a ship's mast and the clouds during a storm, or it can happen when a substance is so hot that its electrons simply can't be permanently anchored by protons anymore. It takes a lot to do this.
One of the things it takes is a reckless disregard for manufacturer's instructions. Microwaves, when you receive them, generally come with a warning. Do not run them if there is nothing inside. If you want to test a microwave, put a cup of water inside. The electromagnetic waves inside a microwave cause water molecules to oscillate and heat, but those waves must have a substance to work on. The microwave oven is designed to have food absorb some of the energy it is putting out. If nothing is there, it's a little like blocking the tube of a bicycle pump. The energized air (or waves) will get back into the device putting out all the energy, and damage it.
Single grapes don't absorb enough power, which is why this experiment can damage your microwave. (It doesn't necessarily damage the microwave every time, but it certainly can.) The grapes also act as dielectric spheres — little antennas that concentrate the power of the electromagnetic waves coming into contact with them. A grape forms a dipole, a structure with constantly circulating electric loop. It also heats up massively, shooting out steam.
To make plasma from all of this, start by slicing the grape. If the grape is left intact, heat will build up inside it, straining against the skin, and it will explode, pre-plasma. Some experiments show it sliced in half. Others show it sliced in half, one half discarded, and the remaining half sliced in half again. Always, though, a little bridge of skin is left between the two soon-to-be-nuked halves. Dry the excess wetness off the cut surfaces of the grape with a paper towel. Put the two halves in the microwave. (Prepare the microwave by taking out the dish that rotates and, if you're being very careful, finding the 'hotspots' in the microwave, where the waves are at their strongest.) Set the timer for no more than ten seconds.
At first the grapes will act like good little antennas, heating up massively, shooting out steam, and forming a long dipole. The skin bridge between them will allow charge to flow. But as the grape loses moisture, the bridge will dry, and become less of a conductor. It may even snap by the end of the ten seconds. The steam above the grape is now hot and electrically charged enough to form plasma. Plasma, by definition, has electrons that are ripped away from their nuclei, and free to move back and forth. This makes it an excellent conductor. The electrons will carry charge between the two grapes, sparking visibly in the microwave.
The sparks should be small and brief, but they give testament to a difference in potential of 3000 volts being closed over a very short time. There will also probably flames, eventually. Cranberries, by the way, also work, but they're harder to find, not as juicy, and create smaller sparks with more chance of damage to the microwave.
Image: Nevit Dilmen