Microwaved grapes make fireballs, and scientists now know why

Here’s a recipe for homemade plasma: Cut a grape in half, leaving the two sections connected at one end by the grape’s thin skin. Heat the fruit in a microwave for a few seconds. Then, boom: From the grape erupts a small plasma fireball — a hot mixture of electrons and electrically charged atoms, or ions.

This trick has been floating around the internet for decades, and previous explanations of the effect have focused on the importance of the connecting skin. But two whole grapes bumped up against one another do the same thing, as do similarly sized waterlogged beads called hydrogels, researchers report in the March 5 Proceedings of the National Academy of Sciences.

The team determined that the grapes act as resonators for the microwave radiation, much like a flute resonates with sound waves. A single grape is just the right size that the electromagnetic waves get trapped within the fruit, bouncing back and forth. Using thermal imaging, the researchers showed that a hot spot appears in the grape’s center, as a result of the trapped radiation. But if two grapes sit next to each other, that hot spot forms where the grapes touch, and salts within the grape skin are ionized and released, producing a plasma flare.

The effect makes for a spectacular kitchen fireworks display, but one you may not want to try at home — it could damage your microwave.