It actually works! I tried this a long time ago with a whole grape and no glass cover. It didn’t work, but maybe I was too impatient.
Cut a grape in half. Eat half of the grape. Cut the second half in half but not all the way through so the grape skin holds the two quarters together. Put it in the microwave and cover with a juice glass.
At about 4 minutes my glass stared to glow from the inside. It was clearly a dancing gas glowing. After 4 minutes and 30 seconds the glass began to melt. Pretty cool! (Or hot.)
The internet says that the glow is from the forth state of matter, plasma.
“Plasma (from Greek πλάσμα, "anything formed") is one of the four fundamental states of matter (the others being solid, liquid, and gas). It comprises the major component of the Sun. Heating a gas may ionize its molecules or atoms (reducing or increasing the number of electrons in them), thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions. Ionization can be induced by other means, such as strong electromagnetic field applied with a laser or microwave generator.” Wikipedia
CAUTION: THE GLASS GETS EXTREMELY HOT AND MAY BEGIN TO MELT.