This is what happens when you microwave a sliced grape.
The microwaves pass through the tiny flap of skin, connecting the two halves of the grape. Once the tiny flap dries out, the waves change and move through the air, creating a spark.
The spark is a result of the electrons being ripped away from the atoms by the wave. These free electrons can compound on themselves, through a process called 'runaway breakdown,' and cause a massive surge of plasma.