Creating plasma using a grape and a microwave is one of the great home science experiments. In this video Veritasium shows you how and discusses the science.
For the confused, the plasma made here is the fourth state of matter, after solids, liquids and gas. It shouldn’t be confused with blood plasma, which is actually a liquid and something you probably shouldn't be experimenting with at home.
That leaves the question, why grapes? As Derek and Steve note, lots of things will work, and you can have plenty of fun finding out which ones do. However, a typical large grape is about a quarter the wavelength of the microwaves your oven produces, which is the perfect size to act as an antenna. The grape needs to be cut so that the joining section heats up to the point where it vaporises while te rest of the grape is still intact.