A bisected grape in the microwave makes plasma. But how does it work? A grape is the right size and refractive index to trap microwaves inside it. When you place two (or two halves) close together the fields interact with each other creating a maximum of electromagnetic energy where they touch. This creates heating, sparks, and plasma, which is further fed with energy directly by the microwaves.
Huge thanks to Hamza Khattak, Prof. Pablo Bianucci and Prof. Aaron Slepkov (unavailable for the call) for chatting to me and helping me understand the physics of this cool phenomenon.