The internet is full of videos of thoughtful people setting things on fire. Here’s a perennial favorite: Cleave a grape in half, leaving a little skin connecting the two hemispheres. Blitz it in the microwave for five seconds. For one glorious moment, the grape halves will produce a fireball unfit for domestic life.

Physicist Stephen Bosi tried the experiment back in 2011 for the YouTube channel Veritasium, in the physics department’s break room at the University of Sydney. On camera, he and the show’s host whooped in the glow of the grape. “Who needs drugs?” Bosi yells in the video, as a particularly psychedelic plume erupts. Off-camera, they discovered they had burned the interior of the physics department microwave.

Hamza Khattak

It’s a crowd pleaser, as long as you avoid melting your kitchen appliances. But it turns out, even after millions of YouTube views and probably tens of scorched microwaves, no one knew exactly why the fireball forms. Popular online explanations usually say that the grape halves act like an antenna, and they somehow direct microwaves onto the small bridge of skin to ignite the initial spark. But nobody had actually done the math to prove it. After several summers of microwaving grape-shaped objects and simulating the microwaving of those objects, a trio of physicists in Canada may have finally figured it out.

The physicists point out that people have let themselves be distracted by the fireball: It’s “exciting and memorable” but “of secondary interest,” they write in a paper that appears today in the Proceedings of the National Academy of Sciences. The fireball is merely a beautiful, hot blob of loose electrons and ions known as a plasma. The most interesting science is contained in the steps leading up to the plasma, they say. The real question is how the grape got hot enough to produce the plasma in the first place.

So 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.