One may think of microwaves in the oven as light shining through translucent food, with the light that gets absorbed being turned into heat. This is a good description of what happens when the food is a continuous mass measuring more than about 6 centimetres in all directions.

However, if the food mass is smaller and oddly shaped, the interaction between the wavelength of the radiation (usually around 12.24 centimetres) and the shape becomes dominant. Microwaves then behave more like radio waves, and some slender, spiky or complex-shaped vegetables become electrical conductors. These conductors act like radio aerials along which electric charges surge back and forth, typically at about 2.5 billion times a second. Wherever these "aerials" form small gaps or fine contacts, electrons leap across, creating hot spots or sparks that scorch the food. This is why conductors such as forks, plates decorated with metal leaf, or unsuitably shaped foods cause arcing.

If your broccoli is in firm contact with the lumps of sweet potato or thoroughly wet, the fluid clogs the gaps and masks any fine tips, preventing harm, but wherever the electrons can arc across fine contacts or gaps, you get sparks or charring.

Jon Richfield, Somerset West, South Africa