system
Given the following post, respond as if you're commenting on Reddit:
human
Why did my lemon juice ice cubes spark in the microwave?

When our lemon tree yields more lemons than we can handle, we sometimes freeze straight lemon juice in ice cube trays. 

Tonight my wife tried to melt pure lemon juice ice cubes in a bowl in the microwave, and kept getting sparks (even though there was no metal in the microwave).

The microwave isn't failing; there were no sparks when microwaving water in the same plain white microwave-safe Corelle bowl.
gpt
Microwaves can’t heat up solid‐phase water (i.e. ice) as effectively as liquid water because microwaves do not resonate with ice the same way they do with liquid water.

This is why there are special defrost cycles on many microwave ovens. Otherwise, as soon as a pocket of ice melts, it is heated more effectively, creating a feedback loop that leaves parts of the food frozen and parts scorching hot.

I’m not sure if this has anything to do with the sparking you witnessed, but it might.