o they revved up their microwave, grape after grape. They made grape replicas out of plastic beads soaked in water. They figured out a way to microwave the grape and its replicas right to the brink of plasma formation, but never letting the sparks actually fly. They even replaced the door to their microwave with a more transparent mesh so they could film the grapes more clearly.

Their conclusions: The grape is less like an antenna and more like a trombone, though for microwaves instead of sound. When you play a trombone, you push vibrating air into it.

The trombone will only sustain vibrations of a particular wavelength—the musical note you hear—depending on where you’ve positioned the slide. Only certain wavelengths, known as standing waves, fit perfectly inside the trombone. As vibrating air of various wavelengths enter the trombone, the standing waves add constructively, while other wavelengths cancel each other out. In other words, the trombone amplifies the standing waves and mutes all others.

The grape, incidentally, is the perfect size for amplifying the microwaves that your kitchen machine radiates. The appliance pushes microwaves into the two grape halves, where the waves bounce around and add constructively to focus the energy to a spot on the skin. Both grape halves happen to focus the energy to the same tiny point. That intense energy jostles the atoms and molecules at that spot, heating them up so much that they can no longer hold onto their electrons, which turns them into a plasma—and boom, fireball.

o they revved up their microwave, grape after grape. They made grape replicas out of plastic beads s