Cut a grape in half but leave the skin of one side intact, Place the grape in the microwave and crank up the microwave radiation. Viola, the grape bursts into a brilliant, white-hot flame. The same effect occurs with two grapes in a microwave that touch each other. The flame you see is plasma – a hot mixture of electrons and electrically charged atoms, or ions.
Scientists only recently discovered why the grapes burst into flames. They used to think grapes acted like antennas, collecting microwaves. Now they understand that grapes in a microwave act as resonators, not antennas. Like a musical instrument that resonant sound waves, a grape in a microwave resonates microwave radiation. In other words, the waves of radiation get trapped inside the grape and bounce back and forth. This forms a hotspot in the center of the grape.
If a grape is cut in half, or two grapes are placed in the microwave side-by-side, this hotspot forms where the grapes touch. The grapes act as a sort of resonator, bouncing and amplifying microwaves between the two halves. Because of the amplified microwaves, electrolytes within the grape skin become supercharged and form plasma, a hot blob of loose electrons and ions which bursts into a bright, violent fireball.
Interesting note – the effect happens with anything grape-size and watery. Gooseberries, blackberries, and even quail eggs produce the same plasma fireball.
Be forewarned: if you try this at home, the flames can burn the top of your microwave.