Why beets and kale can catch fire in the microwave

Most people know not to put metal in the microwave. (Watch the above video, but don't try it yourself.) Even the filigree in an old bowl can spark and ignite a kitchen fire. That's because the metal reflects the microwaves. But what's the deal with the occasional vegetable fire in the microwave? After a beet burn in his family's microwave, scientist Sam Westreich investigated and posted his findings to Sharing Science:

1. Many vegetables, including spinach, kale, dark leafy greens — and beets — are high in iron.

2. Because plants take up iron and other minerals from the soil, these elements tend to be most concentrated in the roots.

Beets are a component of a dark, leafy, iron-rich plant (#1), and we're microwaving the roots (#2).

Additionally, we had a couple other contributing factors: we were microwaving small chunks of beet, which meant lots of little air gaps for sparks to jump, and the beets didn't have a sauce or other liquid to absorb the microwave energy.

And it turns out that you don't need a lot of iron to get a spark in the microwave. Just a small bit of iron acts as a focal point, almost like a lightning rod, leading to charge building up in the surrounding material. A little bit of iron on the edge of a cut piece of beet, and the charge will build up until it arcs — causing char and smoke in the surrounding vegetable as that little bit of iron grows very hot.