If you take a grape and cut it almost completely in half and put it in the microwave, you can make some plasma! Go to the laws of physics, yeah! Oh yeah, it’s still going. You don’t need to cut the grapes in half; two grapes placed side by side will make this effect work, as long as they stay in contact. Now, with the very strong electromagnetic fields at the intersection of the two grapes, what you can get is some sparks and breakdown of air. This is because the electric fields are strong enough to ionize the air, creating those sparks and leading to the plasma. It creates these ions which can then receive more energy from the microwaves. You can see it here pulsing at 120 hertz, which is twice the 60 hertz frequency of mains power and reflects the fact that the amplitude of the microwaves is pulsing twice every cycle.